This tutorial shows how to create and validate Discount Codes (template shared below).
Let's assume our Discount Code is COUPON123.
Now:
1- Wherever you choose, add an Expression card
2- Type in "If last message is COUPON123" and let the AI do the work for you
3- Add an "always" condition after that, in case the user gets the code wrong
Now you'll have 2 different paths, you can send who used the right code to a discounted payment link (template below).