In The Crown Tundra area, Pokémon players can grab some new accessories to customize their in-game avatar. One of those items is a Hex Nut Sport Cap, and you should be able to get this hat rather quickly once you make it to the new area.
To unlock this hat, you will first need to talk to the scientist that is at The Crown Tundra Train Station. However, for her to give you the hat, you will need either Meltan or Melmetal in your party once you begin the conversation. That shouldn’t be much of a surprise, given that Melmetal is the Hex Nut Pokémon after all.
Once you meet this objective, she will ask you to show her the Pokémon. After you do this, the scientist will reward you with the Hex Nut Sport Cap.