I don't even see that promotion under Benefits.
This promotion is quite confusing.
So I did some Google searching and found this about the promotion:
Galaxy Fold and Galaxy Tab S5e devices will also come with a 4-month offer once available for purchase. And all other Samsung Galaxy device owners who activate a device between February 20, 2019 - February 29, 2020 will be able to try YouTube Premium for 2 months free*. Both 4- and 2-month offers are available globally where Samsung Galaxy devices and YouTube Premium are available.
Here the official info about the promotion: