There is an option in the product page to "Create an affiliate link with tracking ID".
You can access this from the Affiliate Dashboard - click the Products tab - click View on the product you want to add the tracking ID to - then you will see the section above with the "Create affiliate link with tracking ID" - click this button and enter your own tracking ID, press Save and then you will be given a unique URL with both your affiliate ID and the tracking ID you have defined.
