1. The ObviousMake sure you have a My Nintendo account with your Switch linked to it.
2. Making sure Smash Bros is Linked to your accountThis first thing you have to do is link your game to your Nintendo Account. If you bought digital, skip to the next paragraph because your game is automatically linked. For physical copy owners of SSBU, go to the home menu. Insert your cartridge into the Switch, if you haven't already, hover over to the cart, and press "+". There will be four options on the left, scroll down to the bottom one "My Nintendo Rewards", and select link game.
Now that your game is linked, we wait. Nintendo will email you a code in up to 10 days (I got mine in 3 I think).
3. If you can't find your codeBut wait there's more! The code may actually show up in the "promotions" folder of your email, or if you have Nintendo newsletters in your spam it may show up there instead.
So check every folder of your email, because you may lose your free Plant code, and we all know he is going to be S tier.
4. Using your codeFinally, once you have the code, type it into the eShop under "download codes", and receive the code for the Plant.