There are two options to activate premium features on AdGuard for iOS app:
1) Purchase a subscription. Just tap on the Get Premium plaque anywhere in the app and follow the on-screen instructions. All you'll need to do is enter your Apple ID password and confirm the purchase. You can choose between a monthly, yearly and lifetime subscriptions.
2) Use an AdGuard license. Log into your AdGuard personal account via the app: go to AdGuard app > Settings tab > License screen and tap on the Login button there. You'll be asked to enter your AdGuard Personal account credentials. After you do, if you have any valid license key in your account, it will be automatically picked up to activate Premium in your AdGuard for iOS app.
As an alternative, you can enter any valid license key in the e-mail field directly to activate Premium features.
PS: AdGuard Pro for iOS (our other iOS app) can only be purchased from App Store.