Option Explicit
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

Public Function DropDown(ByRef obj As Object)
Dim ret As Long
    ret = SendMessage(obj.hwnd, CB_SHOWDROPDOWN,  1 , ByVal  0 &)
End Function

Private Sub Command1_Click()
    Me.DropDown Me.Combo1
End Sub

Private Sub Form_Load()
Dim i As Integer

With Me.Combo1
    .Clear
    For i =  1  To  10 
        .AddItem "Item " & i
    Next i
    .ListIndex =  0 
End With

End Sub