See also: Facebook documentation
To operate a Facebook Live stream, you must have control over a Page.
First, open your Facebook Page. Click on "Publishing Tools":
On the left part, click on "Video Library":
Now you can choose to create your Live:
You get a new window, offering you a streaming configuration:
You will have to copy the "Server URL" and the "Stream key" in the Multicam configuration.
Important: DO NOT CLOSE THE WINDOW THROUGHOUT THE DURATION OF THE SETTING! OTHERWISE, YOU WILL HAVE A NEW CONFIGURATION PROPOSED BY FACEBOOK.