It will automatically present that option when you open the app and were not a subscriber or had a free trail earlier. Obviously you can try with a new Google account if you still want to take the trial.
* With eligible purchase. New customers of YouTube Premium, YouTube Music Premium, YouTube Red and Google Play Music only. Redeem in the YouTube application on eligible devices between February 14, 2020 and March 5, 2021. Form of payment required. $11.99/mo after trial.
Full terms:
https://www.youtube.com/t/premium_restrictions.