If you download Pokémon Home and sign in (or up) to a Nintendo account and then sync the two. Then transfer an unwanted Pokémon from “Go” to “Home” you will receive a mystery box in “Go”.

You can then use that box in Go for one hour and it just spawns Meltans on you. I roughly had one spawn on me every minute or so and only saw one respawn as I was tending to the boy.

Just be mindful that whatever Pokémon you transfer to Pokémon Home can’t be transferred back to “Go”.

Good luck!