Some apps and devices allow you to stream to Twitch. If they require a stream key, you can find it in your account's stream settings.

Most streaming apps will ask you for your Twitch stream key before they let you go live this is how the apps know what account to broadcast to. Some games also let you enter your Twitch stream key ...

Finding a Twitch stream key is necessary if you want to go live on Twitch and here's a comprehensive guide for the same.