Try the 'Not In List' event... ie...
Private Sub id_string_NotInList(NewData As String, Response As Integer)
If MsgBox("The Value -- " & NewData & " -- that you have entered into the " _
& "Purchase Order Type field " _
& "is currently not a valid PO Type. Would you like to add it to the List " _
& "for future use?", vbYesNo, "New Purchase Order Type?") = vbYes Then
Response = acDataErrAdded
DoCmd.SetWarnings False
DoCmd.RunSQL "INSERT INTO tblPrefix (prefix) VALUES ('" & NewData & "');"
DoCmd.SetWarnings True
Else
Response = acDataErrContinue
Me.id_string.Undo
End If
End Sub