Note the fact github uses their icon to link back to the primary domain index page from sub-domains.
A word of advice: don't do it the way every other shop does it, with a little field that says "promo code" that laughs in the face of every single paying customer that doesn't have a promo code. It will take users out of the payment flow and off to google to try and find a promo code, and your abandonment rate will go up (plus you'll be giving discounts to people who wouldn't have even known there was a discount to have until you waved it in their face.)
Instead, make promo landing pages that set a cookie that will add a discount to the next order that visitor makes, and distribute the URLs, not the coupon code.