Apple iPhone – in the app
If you
HAVE an active subscription, tap the icon labelled "More"on the main navigation bar of the app
DON'T HAVE an active subscription, follow the steps when opening the app until you are on the "Subscription page", where a plan can be selected
Look for "Redeem Code" further down the page and tap on it
A window opens, enter the code in the input field and press the button
If the offer code is valid, the offer will be applied. If not, you will see an error message with further details.
Apple iPhone – through App Store
Open the App Store: Click the App Store icon or find it via Spotlight Search.
Sign In: Ensure you are signed in with your Apple ID.
Click Your Name or Profile: Located at the top-right corner of the App Store window.
Select “Redeem Gift Card or Code”: A new window will appear.
Enter the Code.
Tap "Confirm"
See also here
Android Phone – through Play Store
On your Android device, open the Google Play app.
At the top right, tap your profile picture.
Tap Payments & subscriptions.
Tap Redeem code.
Enter the code.
Tap Redeem.
Tap Confirm.
See also here