You can go to one of your "Scenes".
And click on "Add".

Select "Add Screen Region".
Left click and hold (in the top left corner), and drag down to the bottom right corner, then release the left mouse button.

Once you have your screen region, you can right click on it to edit it.
Including the X/Y, of its top left corner.
And its total width and height.

I've got the full 1920x1080 option and the 1920x1040, for my Left Monitor.
Essentially, the whole monitor or just the Warcraft window.

Right monitor is another scene.
As is "Both Monitors".

And then three "Cover up the screen" scenes.
For when I'm entering my account/password, or otherwise want to be private.