ReplayKit: Select window to record?

I have a simple macOS app, with two windows. I would like the user to be able recored each window using a button on the window.

I can record the main window and save that to a file using ReplayKit - so far, so good.

But I can only record the main window - even if I have all ReplayKit code in the second NSViewController, its the main widow thats being recorded.

It is possible to tell ReplayKit what window to record somehow?