Automatically assigning affiliate code to sale

I have set up my store with the affiliate system and I understand how it works (you have to add the affiliates, then set up text/graphic banners, etc). I have also tested it and it works correctly - if the product is sold through the banner, the sale is registered to the respective affiliate.

However I have a product whose sales I would like to assign to a particular affiliate, even if such product is not bought using the banners (ie. people go to the store and then search for the product, or they arrive at it via search engines).

Is this possible? So far I’ve been manually editing the orders to add the affiliate code, but that procedure alone takes time and I would like to automate it.

If this is not possible, then could I at least pass the affiliate code via an URL parameter (ie.