Send Pokémon to GO Parks in ‘Pokémon: Let’s Go’ Trainers with Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee! can send Pokémon from Pokémon GO to their Nintendo Switch. Just like transferring Pokémon to Professor Willow, Trainers earn Candy for each Pokémon they send to their Nintendo Switch. These Pokémon will appear in the GO Park complex in Pokémon: Let’s Go. As a bonus for sending Pokémon to the Switch, you’ll earn additional XP and you’ll unlock the special Mystery Box item the very first time you send a Pokémon. The Mystery Box allows you to catch the Mythical Pokémon Meltan.
Pairing and connecting to the Nintendo Switch
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button. Once the connection is complete, the white Nintendo Switch icon will appear under the Available Devices section. You can now send Pokémon to your Nintendo Switch.
Sending Pokémon to the GO Park Complex
After you’ve paired Pokémon GO to your Nintendo Switch, you can send Pokémon to the GO Park complex in Pokémon: Let’s Go. Pokémon that you’ve brought from Pokémon GO into Pokémon: Let’s Go can’t be transferred back to Pokémon GO.
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button. 
Once sent, your Pokémon will appear in the GO Park complex. You’ll earn Candy for each Pokémon you send as well as bonus XP. The first time you send a Pokémon to the Switch, you’ll also receive a Mystery Box in your inventory. There’s no limit to the number of Pokémon GO accounts that can be tied to a single save file in Pokémon: Let’s Go. That means you’re not limited to connecting with just your own Pokémon GO account. You can receive Pokémon from friends and family’s Pokémon GO accounts.
Caution:
While transferring a Pokémon from the smart device app Pokémon GO to Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee! on Nintendo Switch, the app may not be able to resume the transfer if the connection is interrupted. However, users will be able to continue to play the game once they exit the transfer screen. Important:
- If the Pokémon GO app gets disconnected while transferring Pokémon to your Nintendo Switch console, there is a chance that the "resume transfer" function will no longer work.
- It is also possible that the Pokémon GO app in which this issue occurred will no longer connect to any supported games on Nintendo Switch.
- To prevent this issue from occurring, ensure that you have a stable connection between the smart device and the Nintendo Switch game when transferring Pokémon.
What to Do: Uninstall the Pokémon GO app from your smart device and then reinstall it. Note: The Pokémon may become lost and cannot be recovered, as uninstalling and reinstalling the app would cause the transfer process to be stuck in limbo. Best Practices: While transferring Pokémon between Pokémon GO and a supported game on Nintendo Switch, we recommend the following to avoid interrupting the connection between the two:
- Do not press the HOME Button or the POWER Button on the console.
- Do not pause the app, press the home button, or turn off Bluetooth settings on the smart device.
- Keep a short distance between the Nintendo Switch console and the smart device.
Opening your Mystery Box
The Mystery Box is a special item in Pokémon GO that temporarily makes the Pokémon Meltan appear more often in the wild. After you’ve used the Mystery Box, you have to wait another 7 days and send another Pokémon to the Nintendo Switch to use it again. 
If you don’t have a Nintendo Switch, you can still obtain a Mystery Box and catch Meltan by sending Pokémon to a friend’s Nintendo Switch game. Additionally, a friend with Meltan can also trade the Pokémon with you. For more information about Mystery Boxes and catching Meltan, visit The Pokémon Company’s website.
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Pokémon GO Game GUIDE