There are two options to activate premium features on AdGuard for iOS app:
1) Purchase the subscription. Just tap on the "Get Subscription" plaque anywhere in the app and you'll be prompted to activate a monthly subscription. All you need to do is enter your Apple ID password and confirm the purchase.
2) Use AdGuard license that you already have. Log into your AdGuard personal account: go to Settings > About > 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 a valid and unused Premium, Standard or Mobile license in your account, it will be used to activate Premium in your AdGuard for iOS app.
As an alternative, you can enter any unused 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.