First, you'll need to register an app with Twitch and get the Client ID and Client Secret.

You can do this from the Connections Tab, under Settings. At the bottom, there's a button to register a new application.

In the "Redirect URI" field, you'll need to type in the URL to your Multistreamer install, with /auth/twitch, ie:

https://example.com/auth/twitch

or

https://example.com/<some-prefix>/auth/twitch

if you're running Multistreamer under some prefix.

Once you've gotten your Client ID and Key, add them to the twitch key of the networks table. You'll also need to set the ingest_server parameter to the RTMP url for a Twitch ingest server. You can find a handy list of URLs at https://bashtech.net/twitch/ingest.php - you'll want to set this to a server near the machine running multistreamer.

...
networks = {
  twitch = {
    client_id = 'client_id_goes_here',
    client_secret = 'client_secret_goes_here',
    ingest_server = 'rtmp://live-dfw.twitch.tv/app',  -- dallas
  },
},
...