How to draw rectangle with diagonal given?

Hello Sir, this is really nice that you are giving this much of knowledge to us without fees or other benefit. Thank you so much for it.

I have query regarding rectangle tool. In video Sir drew a rectangle with diagonal assumed to be given as 2 but what command did he use, I didnt get. I tried on the basis of his verbal instructions but it not drawing rectangle with diagonal. Please guide me with the detailed steps for the same.