It's easy. You set MainStage to use your audio interface, the same way you would Logic Pro. Open Google Chrome and go to Streamyard.com and create can account. Once you are signed up, (and it's free f0r basics) log in and tell it whether you want to Live Stream, Record, or do an On Air Webinar Once you choose one, maybe name it test, you will see settings for your audio and video devices, just like Zoom, or any other.
it's way less complicated than OBS.