How do I make a 2 point corner with a rectangle and keep the right thickness?

  angle, c++, math, sfml

I’m trying to draw a dialog box with sfml and I’m using a VertexArray with quads. I want it to have a corner with 2 points like in the image. Could someone help me with the math on how to get the length of A? Or maybe someone has a suggestion to do it in a different way in sfml?

Source: Windows Questions C++