I normally use another way to do the same

using System.Xml;
using System.Net;
using System.IO;

public static void CallWebService()
{
    var _url = "http://xxxxxxxxx/Service1.asmx";
    var _action = "http://xxxxxxxx/Service1.asmx?op=HelloWorld";

    XmlDocument soapEnvelopeXml = CreateSoapEnvelope();
    HttpWebRequest webRequest = CreateWebRequest(_url, _action);
    InsertSoapEnvelopeIntoWebRequest(soapEnvelopeXml, webRequest);

    // begin async call to web request.
    IAsyncResult asyncResult = webRequest.BeginGetResponse(null, null);

    // suspend this thread until call is complete. You might want to
    // do something usefull here like update your UI.
    asyncResult.AsyncWaitHandle.WaitOne();

    // get the response from the completed web request.
    string soapResult;
    using (WebResponse webResponse = webRequest.EndGetResponse(asyncResult))
    {
        using (StreamReader rd = new StreamReader(webResponse.GetResponseStream()))
        {
            soapResult = rd.ReadToEnd();
        }
        Console.Write(soapResult);        
    }
}

private static HttpWebRequest CreateWebRequest(string url, string action)
{
    HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
    webRequest.Headers.Add("SOAPAction", action);
    webRequest.ContentType = "text/xml;charset="utf-8"";
    webRequest.Accept = "text/xml";
    webRequest.Method = "POST";
    return webRequest;
}

private static XmlDocument CreateSoapEnvelope()
{
    XmlDocument soapEnvelopeDocument = new XmlDocument();
    soapEnvelopeDocument.LoadXml(
    @"<SOAP-ENV:Envelope xmlns:SOAP-ENV=""http://schemas.xmlsoap.org/soap/envelope/"" 
               xmlns:xsi=""http://www.w3.org/1999/XMLSchema-instance"" 
               xmlns:xsd=""http://www.w3.org/1999/XMLSchema"">
        <SOAP-ENV:Body>
            <HelloWorld xmlns=""http://tempuri.org/"" 
                SOAP-ENV:encodingStyle=""http://schemas.xmlsoap.org/soap/encoding/"">
                <int1 xsi:type=""xsd:integer"">12</int1>
                <int2 xsi:type=""xsd:integer"">32</int2>
            </HelloWorld>
        </SOAP-ENV:Body>
    </SOAP-ENV:Envelope>");
    return soapEnvelopeDocument;
}

private static void InsertSoapEnvelopeIntoWebRequest(XmlDocument soapEnvelopeXml, HttpWebRequest webRequest)
{
    using (Stream stream = webRequest.GetRequestStream())
    {
        soapEnvelopeXml.Save(stream);
    }
}

I got this simple solution here:

Sending SOAP request and receiving response in .NET 4.0 C# without using the WSDL or proxy classes:

class Program
    {
        /// <summary>
        /// Execute a Soap WebService call
        /// </summary>
        public static void Execute()
        {
            HttpWebRequest request = CreateWebRequest();
            XmlDocument soapEnvelopeXml = new XmlDocument();
            soapEnvelopeXml.LoadXml(@"<?xml version=""1.0"" encoding=""utf-8""?>
                <soap:Envelope xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" xmlns:soap=""http://schemas.xmlsoap.org/soap/envelope/"">
                  <soap:Body>
                    <HelloWorld xmlns=""http://tempuri.org/"" />
                  </soap:Body>
                </soap:Envelope>");

            using (Stream stream = request.GetRequestStream())
            {
                soapEnvelopeXml.Save(stream);
            }

            using (WebResponse response = request.GetResponse())
            {
                using (StreamReader rd = new StreamReader(response.GetResponseStream()))
                {
                    string soapResult = rd.ReadToEnd();
                    Console.WriteLine(soapResult);
                }
            }
        }
        /// <summary>
        /// Create a soap webrequest to [Url]
        /// </summary>
        /// <returns></returns>
        public static HttpWebRequest CreateWebRequest()
        {
            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(@"http://localhost:56405/WebService1.asmx?op=HelloWorld");
            webRequest.Headers.Add(@"SOAP:Action");
            webRequest.ContentType = "text/xml;charset="utf-8"";
            webRequest.Accept = "text/xml";
            webRequest.Method = "POST";
            return webRequest;
        }

        static void Main(string[] args)
        {
            Execute();
        }
    }

I think there is a simpler way:

 public async Task<string> CreateSoapEnvelope()
 {
      string soapString = @"<?xml version=""1.0"" encoding=""utf-8""?>
          <soap:Envelope xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"" xmlns:soap=""http://schemas.xmlsoap.org/soap/envelope/"">
              <soap:Body>
                  <HelloWorld xmlns=""http://tempuri.org/"" />
              </soap:Body>
          </soap:Envelope>";

          HttpResponseMessage response = await PostXmlRequest("your_url_here", soapString);
          string content = await response.Content.ReadAsStringAsync();

      return content;
 }

 public static async Task<HttpResponseMessage> PostXmlRequest(string baseUrl, string xmlString)
 {
      using (var httpClient = new HttpClient())
      {
          var httpContent = new StringContent(xmlString, Encoding.UTF8, "text/xml");
          httpContent.Headers.Add("SOAPAction", "http://tempuri.org/HelloWorld");

          return await httpClient.PostAsync(baseUrl, httpContent);
       }
 }

The best practice is to reference the WSDL and use it like a web service reference.
It’s easier and works better, but if you don’t have the WSDL, the XSD definitions are a good piece of code.

I wrote a more general helper class which accepts a string-based dictionary of custom parameters, so that they can be set by the caller without having to hard-code them. It goes without saying that you should only use such method when you want (or need) to manually issue a SOAP-based web service: in most common scenarios the recommended approach would be using the Web Service WSDL together with the Add Service Reference Visual Studio feature instead.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Xml;

namespace Ryadel.Web.SOAP
{
    /// <summary>
    /// Helper class to send custom SOAP requests.
    /// </summary>
    public static class SOAPHelper
    {
        /// <summary>
        /// Sends a custom sync SOAP request to given URL and receive a request
        /// </summary>
        /// <param name="url">The WebService endpoint URL</param>
        /// <param name="action">The WebService action name</param>
        /// <param name="parameters">A dictionary containing the parameters in a key-value fashion</param>
        /// <param name="soapAction">The SOAPAction value, as specified in the Web Service's WSDL (or NULL to use the url parameter)</param>
        /// <param name="useSOAP12">Set this to TRUE to use the SOAP v1.2 protocol, FALSE to use the SOAP v1.1 (default)</param>
        /// <returns>A string containing the raw Web Service response</returns>
        public static string SendSOAPRequest(string url, string action, Dictionary<string, string> parameters, string soapAction = null, bool useSOAP12 = false)
        {
            // Create the SOAP envelope
            XmlDocument soapEnvelopeXml = new XmlDocument();
            var xmlStr = (useSOAP12)
                ? @"<?xml version=""1.0"" encoding=""utf-8""?>
                    <soap12:Envelope xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance""
                      xmlns:xsd=""http://www.w3.org/2001/XMLSchema""
                      xmlns:soap12=""http://www.w3.org/2003/05/soap-envelope"">
                      <soap12:Body>
                        <{0} xmlns=""{1}"">{2}</{0}>
                      </soap12:Body>
                    </soap12:Envelope>"
                : @"<?xml version=""1.0"" encoding=""utf-8""?>
                    <soap:Envelope xmlns:soap=""http://schemas.xmlsoap.org/soap/envelope/"" 
                        xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" 
                        xmlns:xsd=""http://www.w3.org/2001/XMLSchema"">
                        <soap:Body>
                           <{0} xmlns=""{1}"">{2}</{0}>
                        </soap:Body>
                    </soap:Envelope>";
            string parms = string.Join(string.Empty, parameters.Select(kv => String.Format("<{0}>{1}</{0}>", kv.Key, kv.Value)).ToArray());
            var s = String.Format(xmlStr, action, new Uri(url).GetLeftPart(UriPartial.Authority) + "/", parms);
            soapEnvelopeXml.LoadXml(s);

            // Create the web request
            HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
            webRequest.Headers.Add("SOAPAction", soapAction ?? url);
            webRequest.ContentType = (useSOAP12) ? "application/soap+xml;charset="utf-8"" : "text/xml;charset="utf-8"";
            webRequest.Accept = (useSOAP12) ? "application/soap+xml" : "text/xml";
            webRequest.Method = "POST";

            // Insert SOAP envelope
            using (Stream stream = webRequest.GetRequestStream())
            {
                soapEnvelopeXml.Save(stream);
            }

            // Send request and retrieve result
            string result;
            using (WebResponse response = webRequest.GetResponse())
            {
                using (StreamReader rd = new StreamReader(response.GetResponseStream()))
                {
                    result = rd.ReadToEnd();
                }
            }
            return result;
        }
    }
}

For additional info & details regarding this class you can also read this post on my blog.

So this is my final code after googling for 2 days on how to add a namespace and make soap request along with the SOAP envelope without adding proxy/Service Reference

class Request
{
    public static void Execute(string XML)
    {
        try
        {
            HttpWebRequest request = CreateWebRequest();
            XmlDocument soapEnvelopeXml = new XmlDocument();
            soapEnvelopeXml.LoadXml(AppendEnvelope(AddNamespace(XML)));

            using (Stream stream = request.GetRequestStream())
            {
                soapEnvelopeXml.Save(stream);
            }

            using (WebResponse response = request.GetResponse())
            {
                using (StreamReader rd = new StreamReader(response.GetResponseStream()))
                {
                    string soapResult = rd.ReadToEnd();
                    Console.WriteLine(soapResult);
                }
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
    }

    private static HttpWebRequest CreateWebRequest()
    {
        string ICMURL = System.Configuration.ConfigurationManager.AppSettings.Get("ICMUrl");
        HttpWebRequest webRequest = null;

        try
        {
            webRequest = (HttpWebRequest)WebRequest.Create(ICMURL);
            webRequest.Headers.Add(@"SOAP:Action");
            webRequest.ContentType = "text/xml;charset="utf-8"";
            webRequest.Accept = "text/xml";
            webRequest.Method = "POST";
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
        return webRequest;
    }

    private static string AddNamespace(string XML)
    {
        string result = string.Empty;
        try
        {

            XmlDocument xdoc = new XmlDocument();
            xdoc.LoadXml(XML);

            XmlElement temproot = xdoc.CreateElement("ws", "Request", "http://example.com/");
            temproot.InnerXml = xdoc.DocumentElement.InnerXml;
            result = temproot.OuterXml;

        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }

        return result;
    }

    private static string AppendEnvelope(string data)
    {
        string head= @"<soapenv:Envelope xmlns:soapenv=""http://schemas.xmlsoap.org/soap/envelope/"" ><soapenv:Header/><soapenv:Body>";
        string end = @"</soapenv:Body></soapenv:Envelope>";
        return head + data + end;
    }
}

As an alternative, and pretty close to debiasej approach. Since a SOAP request is just a HTTP request, you can simply perform a GET or POST using with HTTP client, but it’s not mandatory to build SOAP envelope.

Something like this:

using Microsoft.Extensions.Logging;
using System;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

namespace HGF.Infraestructure.Communications
{
    public class SOAPSample
    {
        private readonly IHttpClientFactory _clientFactory;
        private readonly ILogger<DocumentProvider> _logger;

        public SOAPSample(ILogger<DocumentProvider> logger,
                          IHttpClientFactory clientFactory)
        {
            _clientFactory = clientFactory;
            _logger = logger;
        }

        public async Task<string> UsingGet(int value1, int value2)
        {
            try
            {
                var client = _clientFactory.CreateClient();
                var response = await client.GetAsync($"https://hostname.com/webservice.asmx/SampleMethod?value1={value1}&value2={value2}", HttpCompletionOption.ResponseHeadersRead);

                //NULL check, HTTP Status Check....

                return await response.Content.ReadAsStringAsync();
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Oops! Something went wrong");
                return ex.Message;
            }
        }


        public async Task<string> UsingPost(int value1, int value2)
        {
            try
            {
                var content = new StringContent($"value1={value1}&value2={value2}", Encoding.UTF8, "application/x-www-form-urlencoded");

                var client = _clientFactory.CreateClient();
                var response = await client.PostAsync("https://hostname.com/webservice.asmx/SampleMethod", content);

                //NULL check, HTTP Status Check....

                return await response.Content.ReadAsStringAsync();
            }
            catch (Exception ex)
            {
                _logger.LogError(ex, "Oops! Something went wrong");
                return ex.Message;
            }
        }
    }
}

Of course, it depends on your scenario. If the payload is too complex, then this won’t work

Call SOAP webservice in c#

using (var client = new UpdatedOutlookServiceReferenceAPI.OutlookServiceSoapClient("OutlookServiceSoap"))
{
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12;
    var result = client.UploadAttachmentBase64(GUID, FinalFileName, fileURL);

    if (result == true)
    {
        resultFlag = true;
    }
    else
    {
        resultFlag = false;
    }
    LogWriter.LogWrite1("resultFlag : " + resultFlag);
}