Welcome to the forums!
What you’re asking for is possible thanks to how fvwm treats keys in terms of their context. For example, if I wanted to have a key binding (Super + C) to open the
WindowList. I’d do this:
Key C A 4 WindowList
To use the same key to close it, I can tell fvwm to consider the key if it’s on a menu, and close it, hence:
Key C M 4 MenuClose
So in your case, you need two bindings – one in the
A context (anywhere) to open the
WindowList, and another, with the
M context to operate on menus.