The newly discovered shiny form of Meltan may appear when you open a Mystery Box from February 5, 2019 at 1pm PST until March 4, 2019 at 11:59pm PST. During the event, you can open the Mystery Box every three days instead of the usual seven.
The Mystery Box works just like an incense for Meltan; you open it and Meltan will appear around you for 30 minutes. To receive (or re-activate) a Mystery Box you need to connect your cellphone to a Nintendo Switch and send a Pokemon from Pokemon Go to Pokemon Let’s Go Pikachu or Eevee. Meltan evolves into Melmetal using 400 candies, so remember to use Pinap Berries with every throw!