A shortcut to enable/disable external video preview (via Mercury Transmit) already exists: / (slash) on the numerical keypad.
Before using it, make sure that you have specified the monitor or device you want to use in Preferences > Video Preview.
Thanks Tim! I tried the / (slash) on the main keyboard so that's what threw me off. Have a great day!
I got Logitech K380 keyboard without numeric pad.
Is there any other was to enable/disable Mercury transmit by shortcut on PC?
Tried to assign numeric pad / (with temp full profile keyboard) to Fn+F4 custom shortcut over Logitech Options software, but funny thing it registers Numpad / as -, so that option does not work.
I found solution
In "Adobe After Effects 14.1 Win en_US Shortcuts.txt" changed:
"ToggleTransmit" = "(PadSlash)(macControl+Shift+/)"
"ToggleTransmit" = "(PadSlash)(Alt+M)"
so it Alt+M for me.
Keyboard shortcut table is at: