I frequently give out promo codes to users that report bugs, but many users have difficulty with redeeming a gift code or don't understand what they are.
A promo code is a gift code that a developer can give away to certain users so they they can get a paid application without having to purchase it. Using a promo code is the same as having bought an application, except that no money is exchanged and the user cannot leave a review/rating for that application. A promo code 'expires' 28 days after the developer requested it from Apple. That only means that it must be redeemed during that time, If redeemed within the 28 days, the user has the app forever.. It's not a trial period that stops working after a month or anything like that.
Promo codes must be reedemed within 4 weeks (28 days).
An update will render non-reedemed promo codes invalid. For example, you report a bug, and I give you some promo codes in appreciation. I submit an update to correct the bug which also invalidates your promo code(s). Just ask me for some new ones if interested.
1) Open the 'App Store' app
2) Select the 'Apps' tab on the bottom
3) Scroll to the bottom and tap the 'redeem' button
4) Though this only mentions 'gift code', you can enter a promo code here also.
5) Enter code
This was made July 29, 2019. Please notify me if this becomes obsolete with future App Store or iOS updates.