This is process of how to add gift card to apple walletvia iphone
Open the App Store application.
At the top of the screen, tap your picture or the sign-in button.
Decide whether to "Redeem Gift Card or Code." You must log in using your Apple ID if this option is not accessible.
Choose "Use Camera" or "Enter Code Manually" and then adhere to the instructions. The gift card's 16-digit code is located on the back, and you must have access to it.
Click "Done."