In order to stream to Twitch with Mevo, you will need to do so via RTMP.
The RTMP URL for Twitch can vary depending on your location, as they have multiple ingest servers around the world.
Twitch provides a list of RTMP URLs based on location here. Choose the RTMP URL that reflects the closest geographical location to where you are streaming from. Paste it into Livestream's RTMP URL field under Simulcast.
Next, you will need to find your stream key. Log in to your Twitch account and navigate to your Dashboard.
Click Stream Key.
Then click Show Key.
A message emphasizing that your stream key should not be shared with anyone will pop up. Click I Understand.
Your stream key will then populate. Copy it to your clipboard and paste it into the Stream Key field in Livestream's Simulcast menu.
Twitch provides more information about recommended encoder settings here.