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.
It is only by email that you use if you previously used an email for it, you should better make a new gmail and you will get the 4 months. remember youtube premium prizes or cange only works with new emails this is how I do it and I'm going for almost a year free