Sign in to a YouTube app using an activation code by entering the code into YouTube.com/activate on a computer or mobile device. You must be signed in to pair the app with the current account. To complete the process, press Allow Access on the confirmation screen.
Find the activation code by navigating to the YouTube app's sign-in screen. Follow the instructions there to receive a code.
Users with multiple Google identities may be prompted to select one after signing in to the website on a computer or mobile device. Choose the desired identity, as the app only pairs with the currently active one at the time of activation.