Put the following in the combo box "not in list" event.

     

Dim strSql As String
   Dim I As Integer
   Dim Msg As String

   'Exit this sub if the combo box is cleared
   If NewData = "" Then Exit Sub

   Msg = "'" & NewData & "' is not currently in the list." & vbCr & vbCr
   Msg = Msg & "Do you want to add it?"

   I = MsgBox(Msg, vbQuestion + vbYesNo, "Unknown Product type...")
   If I = vbYes Then
       strSql = "Insert Into lkup_login([LoginName]) " & _
                "values ('" & NewData & "');"
       CurrentDb.Execute strSql, dbFailOnError
       Response = acDataErrAdded
   Else
       Response = acDataErrContinue
   End If

strSql = "Insert Into lkup_login([LoginName]) " & _
                "values ('" & NewData & "');"

lkup_login - table name
LoginName - field name

The combo box "limit to list" property is set to "yes".

Let me know!

Kim,
  I think what you really want to do, is that when a CustName does not
appear in a combo box (NotInList), the user should be taken to a blank new
tblCustomer record.  All the new customer info can be updated there
(Address, Zip, Phone...).  After the new customer is added to tblCustomers,
closing that form would return you back to your original "calling" form with
the combo box.
  Given that tblCustomers now has a new customer.. a Requery of the combo
box, would now show that name for selection.

  In other words, new customers are not added to your database via the
combo box, but by entering them into tblCustomers, which in turn is the
RowSource for your selection combo box.