Click your profile icon in the top right then select "settings" from the dropdown
Under settings click the "Channel and Video" subtab and that will take you to a page with your stream key
Click "Copy" to copy your stream key. This is only half of what you need.
The other half is to select a Twitch "ingest server" which can be found here. For simulcasting in Mux any of the US East ingest servers are best, recommend your users use one like Ashburn, Va rtmp://live-iad.twitch.tv/app/
