Each listing can have coupon code. You need to edit listing type post to allow author add coupon code for their listings.
Don’t know what is listing type?
Watch this video guide to understand what does listing type do: https://youtu.be/Tn7-F630lO4
Add Coupon Codes field to submit listing page
Then author can add coupon codes on submit/editing listing page
Display Coupon Code widget on listing page
Edit the listing type post, add Coupon Code element to builder on -> Single tab -> Listing sub-tab
Then showing on listing page