Gurumin announced a code at the end of PM2023 34. For a limited time, viewers could obtain a Terastal Cap in Pokémon Scarlet and Violet from December 22nd, 2023 to November 30th, 2024 at 11:59 JST. The code was: WEARTERASTALCAP

Get a Terastal Cap in Pokémon Scarlet or Pokémon Violet

Use this password to claim a slick cosmetic item for your character to wear.

Show off your love of Tera Pokémon with an excellent new hat your Pokémon Scarlet or Pokémon Violet character can wear. The Sporty Cap (Terastal) in-game item bears a gem-like symbol evocative of a Tera Jewel, and it will make your Trainer look extra stylish as you travel around Paldea, Kitakami, and Blueberry Academy.
Use these steps to claim your in-game gift.

1. Launch your Pokémon Scarlet or Pokémon Violet game.
2. Select Poké Portal on the X menu.
3. Select Mystery Gift, then select Get with Code/Password to connect to the internet.
4. Enter the password WEARTERASTALCAP.
5. Watch as the item arrives in your game.
6. Be sure to save your game.

This password expires on Saturday, November 30, 2024, at 6:59 a.m. PST. Enjoy becoming a true fashion icon, Trainers!

Source: https://www.pokemon.com/us/pokemon-news/get-a-terastal-cap-in-pokemon-scarlet-or-pokemon-violet