Forms :: Auto Complete Field Within Form

I have created 2 tables, one for direct customers and one for indirect customers. (I consider indirect a customers customer). The 2nd table has a field that links every indirect customer to a direct one. I also have a table for inserting credit notes. The credit note is either for a direct or an indirect customer. I am using a form for this one. So the form has 2 fields, one for each kind of customer. If the customer is direct, i only insert the direct customer field. If its an indirect customer i insert the indirect customer field. What i want is to create a combo box that when choosing an indirect customer, to display the linked direct customer and to save it to the direct customer field.