WM_MENUCHAR

  This message is sent to the window who owns the currently active menubar
when an ALT-key combination has been pressed which does not correspond to a
menubar item.

Parameters
  wParam is the code of the key that was pressed. See the file keys.h for a
   list of key codes.
  LOWORD(lParam) is MF_POPUP.
  HIWORD(lParam) is the handle of the menubar.

Returns
  The window procedure should return 0 if the keystroke should be discarded,
1 if the menu should be closed, and a number greater than 1 to represent
the id of a valid menu  item which should be chosen.