Pokemon Co. has detailed a special event for Pokemon: Let's GO in South Korea. Those who pick up the game from participating retailers in May 2019 will get a special serial code for a Shiny Pikachu or Shiny Eevee. Those who already have the game can still get a code from the upcoming Pokemon Festa, which takes place May 3rd to 12th, 2019. You just have to show the staff at the event your Switch, which let's them confirm that you have the game installed. You can also show them a game card or Poke Ball Plus for the code.
For Let's Go Pikachu, you'll get a Shiny Pikachu that's Level 10, and comes with the moves Thunderbolt, Quick Attack, Iron Tail, and Helping Hand. If you have Let's Go Eevee, you'll get a Shiny Eevee that's Level 10, and comes with the moves Double-Edge, Dig, Iron Tail, and Helping Hand.