This method is only relevant to you if you have a switch console and either the Pokemom game Lets go Eevee or Lets Go Pikachu. Players first must have also completed enough of the game to be able to reach Fuschia City. Once you are at this stage here are the following steps required to unlock a mystery box.
Open Pokemon Go
Tap Pokeball Icon to go to Menu
Tap of Gear Item to Open Settings
Scroll down and tap on Nintendo Switch
Tap on connect to Nintendo Switch which will put pokemon go in pairing mode.
In Pokemon: Lets Go, follow the steps to pair the switch with Pokemon go in Settings Menu.
Once the connection has been completed, switch icon will appear under available devices section so you are now ready to send Pokemon to Lets Go by following the next steps
Open Pokemon: Lets Go
Go to Fuschia City and speak to the attendant inside the GO Park Complex and select Bring Pokemon
In Pokemon GO in Menu tap Pokemon Icon to go to Pokemon Storage
Tap on Switch Icon which you will see in top right corner
Select Pokemon youd like to transfer. This is a one way only transfer so select only Pokemon you no longer will use in Pokemon Go. In addition only Pokemon from Kanto Region can be transferred to Lets Go.
Finally Tap Send to Nintendo Switch icon to complete transfer which will then activate mystery box.
For further info please check out Niantic Support
This method once set up is much more convenient to activate a mystery box and most importantly is free as does not require owning a Nintendo Switch as only requires the free app called Pokemom Home.
Open Pokemon Home App
Create a New or Sign into a existing Nintendo account
3.Open Pokemon Go
Tap Pokeball Icon to go to Menu
Tap of Gear Item to Open Settings
Scroll down and tap on Pokemon Home
Sign in using same Nintendo Account associated with your Pokemon Home.
Tap Send Pokemon and select any Pokemon youd like to transfer. This is a one way only transfer so select only Pokemon you no longer will use in Pokemon Go.
Finally tap Transport to send to Pokemon Home which will activate Mystery Box as long as you dont have a pending Pokemon to be collected in Pokemon Home. If this is the case you must first go into Pokemon Home and collect Pokemon before you are able to transport additional Pokemon.
Tip: Free version on Pokemon home has limited spaces so recommend using Pokemon you are happy to release. This can be done in Pokemon Home by Tap and Hold on a Pokemon you want to remove and select release when Pokemon option appears.
For further info please check out Niantic Support