Option Explicit
Private Declare Function SendMessage Lib "user32.dll" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Integer, ByVal lParam As Any) As Long

Public Sub DropDownCombo(cbo As ComboBox)
    Const CB_GETDROPPEDSTATE = &H157
    Const CB_SHOWDROPDOWN = &H14F
    If Not CBool(SendMessage(cbo.hwnd, CB_GETDROPPEDSTATE, ByVal  0 , ByVal  0 &)) Then
        SendMessage cbo.hwnd, CB_SHOWDROPDOWN, True, ByVal  0 &
    End If
End Sub

Private Sub Label1_Click()
    DropDownCombo ComboBox2
End Sub