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
 
Private Const CB_SHOWDROPDOWN = &H14F
 
Private Sub Command1_Click()
   SendMessage Combo1.hwnd, CB_SHOWDROPDOWN, True, 0
End Sub
 
Private Sub Form_Load()
   Combo1.AddItem "Test"
   Combo1.AddItem "Test"
End Sub