Switching between client windows in multiplayer test

Is there a short-cut key to switch between the various client and server windows while testing multi-player games? I can do it with the windows key, but requires a couple of extra clicks to then activate the other window. In the tutorial videos it looks like there’s a magic button which just swaps the controls to the other window.
Cheers.

Also, I forgot you can use the back quote or grave key.

Alt Tab works best.