MS Access: Use NotInList with with a hidden key column

How do I use the OnNotInList event to add a record to a table underlying a combobox rowsource when the first column is a hidden column containing the key field?

The problem is that if the user types a value into the combobox, and I add that value to the underlying record, when access goes to check whether the value has been added it looks for the users entry (which is a string value) in the ID field.

I've tried  having the NotInList subroutine add the record to the underlying table, then changing the value of the newly created key, so that when vba runs the "response = acDataAdded" line,
it will check for the new key instead of the string the user typed, but that doesn't work for some reason.

I don't want to open a new form.  I just want to add the record behind the scenes.

Can anybody help?


Here's my code:

Private Sub cboCompanyID_NotInList(NewData As String, Response As Integer)

    Dim RS As Recordset

    Set RS = CurrentDb.OpenRecordset("tblCompanies", dbOpenDynaset)
    With RS
        .AddNew
        .Fields("Company") = NewData
        .Update
    End With
    cboCompanyID = RS.Fields("ID")
    Response = acDataErrAdded
   
Exit Sub