If it doesn't prompt you automatically, select it manually by clicking the Chromecast icon in your toolbar (or clicking on Cast from the Chrome menu if you do not have the Chromecast extension installed) and selecting the desired device.
TwitchCast - Step 3/3
You don't need to keep this page up for TwitchCast to play. You can change a few things below if you want.
There are a lot of similar projects floating around the internet, but none work how I want them to. None of them integrate chat, and many of them function poorly. Twitch's HLS servers do not send out open CORS headers, so data must be proxied (which can be expensive). This is the likely cause for most of these competing services that glitch and freeze. It is my hope that I can provide a more reliable service for the small percentage of Twitch Chromecast users.