https://www.gamespot.com/articles/pokemon-free-legendaries-available-now-for-ultra-s/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.

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.