From now until 31st March 2025 (or while supplies last), purchasing the upcoming Mario party title from Nintendo of America will net you the download code. Given that the offer is available beyond the game's October launch date, it means you don't need to rush out and pre-order if you want that subscription and you can wait until after launch.
The code will give you three free months of NSO membership or extend your current subscription for another three months. But it's worth noting that if you have a Family Membership or are subscribed to the Expansion Pack, then you can't use the code. This is only for an individual, base-tier subscription.
Super Mario Party Jamboree is the newest Mario Party game, and the third Mario Party title on the Switch. Launching on 17th October 2024, the game will come with seven boards — five brand new ones, two classic returning ones — 110 minigames, and a 20-player online multiplayer mode titled the Koopathlon.
If that last mode has piqued your interest, then you'll need to have that NSO subscription, so the offer is a pretty nice deal if you're planning to play online with others over the holiday period. (We know it's August, we know the holiday's aren't right now... but c'mon, Mario Party + the holidays?)