If the affiliate should link to a particular product's page rather than your Common Hop Link landing page URL, you can set up a product-specific hop link:
http://www.e-junkie.com/ej/help.affiliates.htm#product
We have no way to make an affiliate link automatically apply a discount, so if your discount requires a code, the affiliate would need to mention that discount code on their page where they're posting the hop link and leave it up to buyers to enter the code in their cart.
You could also make the product-specific hop link point to a "secret" landing page for affiliates -- i.e., the page would only be accessible via hop link redirection but not linked from the rest of your site -- so you could put some Add to Cart button(s) on that page which have a discount code built-in:
http://www.e-junkie.com/ej/tips.discounts.in-button.htm
If this 20% discount is a short-term "sale" for all buyers (affiliate referred or not), you might consider configuring the discount without a discount code, so the discount would apply automatically to all orders meeting the discount's Min. Item Total/Qty. thresholds. This help page explains all our discount settings in detail:
http://www.e-junkie.com/ej/help.discounts.htm
