How to redeem an iTunes code.
Launch the App Store. Tap Today at the bottom of your screen. Select your user icon at the top of the screen (or sign in if needed). Tap Redeem Gift Card or Code to enter your code.
Launch the App Store. On the sidebar, click your name or the sign in button. Tap Redeem Gift Card or Code to enter your code.
Launch the Apple Music app. Press the menu button. Hit Account. Choose Redeem Gift Card or Code. Enter your code and tap on Redeem.
Launch iTunes. On the menu bar, choose Account, then pick Redeem. Sign in and follow the on-screen directions.