Discount codes (coupons) may only be used on the website. Apple and Google do not allow the use of coupons.  If you already have a free account (either expired PRO or simply a brand new non PRO account) you must sign in first before upgrading to PRO. If you go through the purchase flow before signing into your free account that you already have, you will end up with two accounts - one PRO and one free.

During the checkout flow one the web (but before you checkout completely) you will see a slot/button to enter a coupon code. Enter it there and then hit APPLY to see the terms before you finish signing up.