How to legally post a video with music in the background on Facebook?

[oh not not again]

Copyright literally means the right to copy. The record labels own the copyrights to the music, not us

So copying it onto the facebook, youtube, anywhere else on the web is a violation of the music's copyrights. copying it for others is a violation of the music's copyrights

you can't do it legally unless the record label licenses it to you. That is if the record label is even willing to license it in the first place, which will cost a lot of money