Players can purchase the digital version of Super Smash Bros. Ultimate or register their physical game card on a Nintendo Switch device by January 31, 2019, to gain access to Piranha Plant when it is available around February 2019 at no additional cost. Piranha Plant, together with a new stage and music tracks, will be available for purchase as DLC in the future. Fans can pre-purchase the game and secure the Piranha Plant reward starting today by visiting http://smashbros.nintendo.com/buy/. Pre-purchase customers have already started to receive their special codes with the below text in a designated email:
Thank you for purchasing the Super Smash Bros. Ultimate game. Below is your download code from My Nintendo for the Piranha Plant in-game character. You can redeem the code now and you’ll receive the new fighter when it becomes available in early 2019.
Source: My Nintendo news