Drawing over an existing background

Hi, just starting out with OpenGL so bear with me…

Can anyone suggest how I can draw OpenGL shapes over the top of an existing window?

I can pass the existing window in to my OpenGL code and it will draw on it but the background (ie. the original window contents) gets cleared to whatever glClearColor has been set to.

Should I be grabbing a copy of the underlying window and the repainting that at some point in the process, or is there a way of using alpha blending to allow the underlying window to ‘bleed through’?

Any suggestions gratefully received!