To further encourage players to use the Pokémon GO to Let’s Go transfer, the first time you transfer a Pokémon from Pokémon GO, you will obtain the “Mystery Box” item in Pokémon GO. You’ll find it in the “Items” menu in your bag, between your incenses and TMs.
Using the Mystery Box
To use the Mystery Box, tap it and select “Open”. Doing so will cause Meltan to appear nearby for 60 minutes. You can expect to find 1 or 2 Meltan every 2 minutes. Note that Meltan is small, so you may need to zoom in a bit or you could miss it. Catching Meltan should be fairly straightforward, but it may be closer than most Pokémon, so you don’t need to throw your Poké Balls as hard.
What to do with Meltan
Once you’ve caught a Meltan or a truckload of them, you’ll probably want to transfer most of them to Let’s Go. This works exactly the same as with any other eligible Pokémon. The act of transferring Meltan will also provide you with additional Meltan candy in Pokémon GO, which will be useful later on.
However, you should keep at least one Meltan in GO. You see, the only way to evolve Meltan into Melmetal is by feeding it Meltan Candy in Pokémon GO (you can’t evolve it in other games). In total, you’ll need 400 Meltan Candy, so go luck with that. Once you’ve evolved Meltan into Melmetal, you can transfer Melmetal to Let’s Go in the same manner.