Discounts for 2020 aren't out yet. They have only published them through 2019. Just need to keep an eye out.
Its not as simple as applying a discount. I don't book packages so I am not sure how it works. But when discounts do come out, in simple terms, you have to kind of rebook. If the room you selected is available for discount, you can then change your reservation.
Just keep in mind that although they publish discounts, you aren't guaranteed. They are dependent on availability.