1. Open Facebook.com on your web browser and log in
  2. Go to the "Go live" section (https://www.facebook.com/live/producer), select "Streaming software" and copy the Stream key: 
  3. Launch OBS Studio
  4. Get the stream of your camera (See Stream an Avonic Camera via RTSP to OBS)
  5. In the top menu, navigate to File > Settings > Stream, select "Facebook Live" as service and paste the Stream key (copied in point 2 above) and click OK 
  6. In the bottom right corner of OBS, click on "Start streaming"
  7. Now you should have the preview on Facebook:
  8.  Choose your settings and preferences (stream now or later, in a group, public/friends, ...) and click the "Go Live" button: you should now be streaming on Facebook from OBS !