But just above that, the Wiki says this:
Choosing "Window" you can select from a number of predefined windows. You cannot use Extensions' windows.
-- from CyberSimian in the UK
Isn't this possible now?
https://www.team-mediaportal.com/wiki/display/MediaPortal1/Mapping+Buttons
Window - select a Window from the dropdown list - the button will then navigate to that...