Lookup field on Add pages

Presently, I have an Add page that allows the user to add a new service item record to the list page. The Customer Name field on the page is a text box, that is marked as required, that I would like to change to a lookup box that would allow the selection of a customer name, if it exists in the table, but also allow a new customer name to be entered that doesn't appear in the list. I was able to change the field on the Add page to do the lookup function successfully, but can't figure out how to get the functionality of being able to enter a name not on the lookup list. Any Ideas? I am using ASPRunner 5.2