[AccessD] Setting default value of a text box

I have a data entry form. The first entry is a list box where the user selects a company name. List box name is companyname. The next data entry is an invoice number. I am trying to set the default value property of the text box using the following statement. The default value is not becoming part of the saved data. Thanks for the help

=IIf([CompanyName].[Value]="Renegade","RWLS",Null)

I also tried
=IIf([CompanyName]="Renegade","RWLS",Null)

Do I maybe need to set it using code in the after update event of the list box?