rightclick > Options > Preferences *) > Key definitions & WinLIRC
There you can define (separately for [Windowed mode] and [Full screen mode]) which key is to be 'Boss button' (for both modes defaultkey is [F10]) as well as define on (3rd) Tab ['Boss' button configuration] what it must do for you.
*):
Base skin has a Preferences button so that you can go here directly