Match Required In Combo Box

I have ComboBoxes in a VBA Form and want to prevent users from typing invalid entries into them (their fields are initially empty).

I want to write a custom function instead of using the MatchRequired property.

Here is a sample of code into which im trying to insert such a solution:


Private Sub cboDomain_Change()

If txtGM2 = "" Then
MsgBox "Please enter GM% first."
End If

Dim strSectorPrefix As String
Dim strSectorName As String
strSectorPrefix = Mid(cboSectorName.Value, 1, 1) ....................................