https://www.polygon.com/2018/10/10/17959490/meltan-how-to-catch-pokemon-go-pokemon-lets-go-transfer

More details about Meltan, the newest addition to the Pokédex, reveal how players will be able to capture the Pokémon once Pokémon: Let’s Go! launches next month. Two videos explain the item necessary for catching the tiny steel-type Pokémon, as well as hint at another big piece of info we have yet to learn about the mythical monster.

The second video in an ongoing series about Professor Willow (of Pokémon Go) and Professor Oak (Pokémon: Let’s Go!) follows the pair as they make another research breakthrough. Meltan, it appears, can only be transferred from Willow’s world into the Kanto region via the Mystery Box, an item that players can only unlock by connecting the mobile game to the Nintendo Switch one.