WM_COMMAND

  This message is sent when a user selects an item from a menu, or when a
control must notify its parent window about an event.

Parameters
  wParam is the id of the menu item selected, or the id of the control.
  lParam is 0 if WM_COMMAND was caused by a menu item selection.
   LOWORD(lParam) is 0 and HIWORD(lParam) is the handle of the control if the
  WM_COMMAND message  was  sent  by  a  control.