You cannot have subscribed to this service in the past, and is technically not even a samsung promotion. it is redeemed within the youtube app. The link above details how to redeem it.
As the subject says, I have my S21 right now but when I go to try and obtain the free trial all it shows me is the 2-Month free trial instead of 4. Never once had any of the youtube/google subscriptions so this is pretty odd.