Go to the Marketing tab in the Seller Hub:
https://www.ebay.co.uk/sh/ovw
there select Promotions.
This opens a new window or tab, select Create a Promotion then Coupons.
Set the discount and conditions of your offer.
Only eligible items will comply. You may need a PC / laptop to do this, Chrome and Firefox work best with eBay.
@jnrich1966