I want to change it from cmd-shift-u to an F-Key. How do I do that?
You could record an action, but you would probably need to two actions, one for on and one for off.
Then go to the action options to assign an F key.
It also might be possible for a script to do a toggle on/off.
On the windows version it doesn't look that is a keyboard shortcut that can be changed in the Edit>Keyboard Shortcuts dialog.