Sorry I wasn't very clear on the Purchase Order question.

I am in Invoices and am going to produce an invoice for a customer. This customer uses the same purchase order number for each order and needs for this purchase order number to appear on all of their invoices. There is a space on the invoice for Purchase Order Number and right now we enter this number manually for each invoice we produce. We have a few customers already set up where their Purchase Order number is entered in their file in Invoices and automatically is entered into the Purchase Order Number space on the printed invoice, however I cannot find where to do this so it automatically fills in on all customers who require it with their specific purchase order number.