you have to go in the options from BlizzInterface by Pressing ESC (Default) Then Keybindings (Don't know right term) there the last one under Addons I thnik it's other ore more (have german client thats why I don't know the right term and then the NxTOGGLEWATCHMINI is your Keystroke to define
Greetings Samyonair
PS for better understanding