https://www.gamespot.com/articles/last-chance-free-legendary-pokemon-available-now-f/1100-6458735/

A new distribution is now underway for Pokemon Ultra Sun and Ultra Moon. From May 4-27, players will be able to add one of the Legendary Pokemon from the Kalos region, Xerneas and Yveltal, to their teams via a free download code. [Update: The Xerneas/Yveltal distribution ends on Sunday, May 27, making this your last chance to pick up a free download code for the Legendaries.]

To claim the download code, players will need to head to a participating retailer and pick one up near the register. In the US, the code will be available from GameStop; UK and Ireland residents can get one from Game stores; and players in Canada and Australia will find the code at EB Games.