Yep, you just have to get your My Nintendo Rewards gold coins by highlighting the game with the card inserted and hitting the + button. Choose registration option in that menu. Nintendo will email the code to you (already got mine).
https://www.destructoid.com/psa-make-sure-you-register-your-physical-copy-of-smash-ultimate-for-free-piranha-plant-dlc-534871.phtml