How to start a new sketch from the corner of a body

I am attempting to extrude a new face that is a half inch wide and aligned with the edge of an existing body. However, when I attempt to draw a half inch horizontal line originating from the top left corner and running along the edge it prevents me from doing it. I have to first draw the line so that it doesn’t overlap the existing edge and then drag it back. Once I do this I’m not able to select the line since it seems to be hidden behind the existing edge. Not sure if this makes sense