If I were using Linux/X, I would be able to tell the window manager to recognise this window and keep it in the corner, or offscreen, or something.
All the web searching I've done comes up with how to keep windows on top - nearly the opposite of what I want.
The closest I've found to a solution is setting Zoom in the Icon Bar to 'Options > Assign to: all desktops' which (bizarrely) hides all the zoom windows underneath all other windows if I'm not actively using it... except, of course, for the one I'd actually like to hide.
Obviously the correct solution is to become massively wealthy, buy Zoom, and force them to fix their damn software. Or buy Apple and force them to rename themselves X and... oh, wait, that's something else.
Is there something I can do to prevent this 'Webinar is almost ready to start' popup from... uh, popping up?