https://www.gamespot.com/articles/last-day-to-register-for-marchs-free-legendary-pok/1100-6457071/

March is nearly here, which means Pokemon Ultra Sun and Ultra Moon players will soon be able to get their hands on the games' next free Legendary Pokemon, Regigigas and Heatran. However, unlike February's free gifts, Dialga and Palkia, The Pokemon Company is distributing the two new Legendaries in North America via its Pokemon Trainer Club newsletter, which you'll need to sign up for by March 1 in order to get them.

To register for the newsletter, you'll need to create a Pokemon Trainer Club account on the official Pokemon website. You can create a free account here. If you're already a member of the website, log in and check if you've opted-in to receive email updates to make sure you don't miss out on the newsletter.