Are you using AffiliateWP but have another WordPress-based site you would like affiliates to promote? If so, then this free add-on is for you. This is exactly what External Referral Links does. It will take the affiliate’s ID or username from the URL and store it as a cookie in the customer’s browser. Any link on site B that links to site A will then have the affiliate ID or username intact.
Sales page/demo: https://affiliatewp.com/add-ons/official-free/external-referral-links/