How do I add the "affilate code" to the checkout form?

Id like customers to be able to enter the 'affilate code' from printed material placed to the customer by the affilate. How do I add that input box to the checkout form?

thanks in advance everyone,


The screen you have shown is for the admin area. To the best of my knowledge, there is no capability for the customer to set an affiliate in the checkout. Normally, the affiliate would be set automatically based on the affiliate banner the customer clicks.