hm not sure.
usually this is combined with a blob detector that finds the outer circle first, and then looks for the corner in its center. you need an easy to find outer circle, so either the yellow marker goes on a dark background, or you put another black circle around the yellow circle so you get a nice contrasting contour.
if you have corners only, the usual trick is to round off those corners you don’t want to be corners. it’ll look like a two-leaf clover.
the corner you want is a “saddle point”. the corner circled in red isn’t a saddle point. this is distinguishable in the second derivative, I think. don’t quote me on it.
you can skip this part for now. it’s a distraction. the homography stuff is the crucial bit. use manually entered coordinates, i.e. hardcoded numbers or have user click on the picture.