Here is how you can use promo codes:
Open the Apple App Store
Scroll to the very bottom of the screen and click redeem
Type in your Apple credentials
Tap on "you can also enter your code manually"
Enter the promo code and tap on "Redeem"
Click "Done"
Also, I read a very helpful article with details of each steps with screenshots for this problem on the following website:
https://www.thelearningapps.com/how-to-redeem-an-inapp-purchase-promo-code-on-iphone-ipad/
ahsanthelearningapps
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