it seems the way to do it is in GO.
> Meltan appears in Pokémon GO. In order to catch Meltan, it is essential to have a Mystery Box, which can be received by sending Pokémon from Pokémon GO to Pokémon: Let's Go, Pikachu! or Pokémon: Let's Go, Eevee!
> Sending Pokémon from Pokémon GO into one of the Nintendo Switch titles will get you certain items in the app, such as Candies for the species of Pokémon you sent. However, the first time you send a Pokémon, you'll also be able to get a very special item: a Mystery Box!
> The Mystery Box closes itself after it's used once. The only way to open it again is to wait a set amount of time, and then send another Pokémon to Pokémon: Let's Go, Pikachu! or Pokémon: Let's Go, Eevee! You can also get a Mystery Box for sending Pokémon to a friend's Nintendo Switch game, but you can only have one Mystery Box at a time!
> Meltan will begin to appear once a Mystery Box is opened in Pokémon GO, but be careful—they'll only appear for the player that opened the Mystery Box. Touch a Meltan you see on the field map, and then try to catch it with a Poké Ball!
> You can then bring the Meltan you catch in Pokémon GO to Pokémon: Let's Go, Pikachu! or Pokémon: Let's Go, Eevee!, where it can follow you around during your adventure in the Kanto region. Even if you don't catch Meltan in Pokémon GO yourself, you can ask a friend to send one that they've caught into your Nintendo Switch game.
Source:
https://pokemonletsgo.pokemon.com/en-us/new-pokemon/