If you have the promo code, you can use that as a free download of the app or unlock In-App Purchase for free.
Official instruction from Apple, how to redeem the promo codes, you can read here: https://support.apple.com/en-us/HT201209
Official instruction from Apple, how to redeem the promo codes, you can read here: https://support.apple.com/en-us/HT201209