How can I make a line feature into a polygon in GRASS GIS by drawing a closed shape? I’m looking for a method to manually edit a vector layer and create a polygon by simply drawing its outline and closing the loop. Is there a specific tool or workflow for this?

I have a heatmap and the four corner coordinates. My task is to mark a new coordinate on the map. The map is not distorted. I am finding the distance of the new point from the left and bottom line is constructed from the top left, bottom left and, bottom right, and bottom left coordinates.