'Deklaration
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal _
  hWnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
 
'Konstante
Private Const CB_SHOWDROPDOWN = &H14F
 
'Aufruf
Call SendMessage(cmb.hWnd, CB_SHOWDROPDOWN, 1, ByVal 0&)