Discuss "Combo Box - Add Entry Not In List" in the Microsoft Access forum.

Private Sub ComboBox_NotInList(NewData As String, Response As Integer)
   Dim ctl As Control
   Dim strSQL As String
   Set ctl = Me!ComboBox

   If MsgBox("Value is not in list. Add it?", _
        vbOKCancel) = vbOK Then

       Response = acDataErrAdded
       
strSQL = "INSERT INTO  ComboListTable(list) VALUES('"
strSQL = strSQL & NewData & "');"
CurrentDb.Execute strSQL

   Else

       Response = acDataErrContinue
       ctl.Undo
   End If

End Sub