Rootless: how to get wm menu?

[Danilo Turina]

With WindowMaker, I select one of the applications running under X and 
then press F12.
I think it's the only way.

 > How do I open the X11 window-manager menu when working in rootless mode?
 > When clicking a mouse button on the background, I get a Windows menu.
 > There should be a small area or special icon as an event proxy for the
 > missing X11 background.