Dim Db As DAO.Database
Dim Rs As DAO.Recordset
Dim Msg As String
Dim NewName As String

On Error GoTo Err_CustomerID_NotInList

    ' Exit this subroutine if the combo box was cleared.
    If NewData = "" Then Exit Sub

    ' Confirm that the user wants to add the new customer.
    Msg = "'" & NewData & "' is not in the list." & vbCr & vbCr
    Msg = Msg & "Do you want to add it?"
    If MsgBox(Msg, vbQuestion + vbYesNo) = vbNo Then
        ' If the user chose not to add a customer, set the Response
        ' argument to suppress an error message and undo changes.
        Response = acDataErrContinue
        ' Display a customized message.
        MsgBox "Please try again."
    Else
        ' If the user chose to add a new customer, open a recordset
        ' using the Customers table.
        Set Db = CurrentDb
        Set Rs = Db.OpenRecordset("tblRouting", dbOpenDynaset)

        ' Ask the user to input a new Customer ID.
        Msg = "Please enter a Name"
        NewName = InputBox(Msg)
        ' Create a new record.
        Rs.AddNew
        ' Assign the NewID to the CustomerID field.
        Rs![Name] = NewName
        ' Assign the NewData argument to the CompanyName field.
        Rs![RoutingNum] = NewData
        ' Save the record.
        Rs.Update

        ' Set Response argument to indicate that new data is being added.
        Response = acDataErrAdded

    End If

Exit_CustomerID_NotInList:
       Exit Sub
Err_CustomerID_NotInList:
       ' An unexpected error occurred, display the normal error message.
       MsgBox Err.Description
       ' Set the Response argument to suppress an error message and undo
       ' changes.
       Response = acDataErrContinue