If you have affiliate links on your website, you may want to consider a click on those links as a goal completion. Because a click on those links would result in the visitor leaving your site, you’ll need to set up event tracking on any affiliate links you wish to track as goals. Use the form to build GA Event tracking code for an Affiliate Link. Then add the code to the affiliate link to track the event.
After you add the affiliate link event tracking code to your link(s), you may want to setup a goal in Google Analytics that's related to the event. Follow these instructions to setup the goal.