You don't, and your question might imply a mis-understanding
OBS runs on your PC. and can use used to composite (compose) a video (and audio) from multiple sources.
OBS can stream direct to a delivery network (YouTube, Facebook, Twitch, etc) or locally record, or both
So you install OBS on your PC, ideally your normal user account does NOT have local admin rights, so you run OBS from normal user account, then connect (point) OBS to Facebook (or other streaming platform).
There are a number of options for connecting to any stream provider (including Facebook), that gets into platform specifics. In Facebook's example, there are Scheduled and non-scheduled (ad-hoc) livestream options, and Persistent vs non-persistent stream keys. Some stream setups can now be done in OBS. However, as I started with OBS v26, and my workflow would NOT benefit from changing, I setup my streams directly in Facebook's /live/producer page. And copy into OBS a non-persistent stream key per stream... but that is my use case/workflow, other users may reasonably do otherwise.
