The Pokemon Company has announced a new offer for Pokemon Let’s Go Pikachu/Eevee.
Starting 16 November 2018, Pokemon Let’s Go players can visit Pokemon Centers (only within Japan) on their birthday to redeem a free serial code. This serial code can then be used in-game to redeem a Chansey.
However, The Pokemon Company did not mention when this promotion would end. They also did not specify how one’s birthday would be verified.
— ぽけりん＠ポケモンまとめ (@matome_pokemon) October 12, 2018