You can install the Twitch extension directly via Twitch. Here is a direct link to the download:
Alternatively, you can find the extension by going to your creator dashboard, clicking on the extension menu on the left, and either directly searching for Baldur’s Gate 3 or clicking on “extensions for games.”
If you connect your Larian account to Twitch, it will automatically enable Baldur’s Gate 3 extension on your channel and link it to the game. This way, your audience can interact with you directly on the choices you make in the game!