you can do so by clicking the "video button" in the reply menu after clicking this button, simply add the link to the video. That, or you can copy/paste the video's url and manually add [video] tags around it.
To post a video: [noparse][video]https://www.youtube.com/watch?v=QBZKyd3yIQc[/video][/noparse] Result: [video=youtube;QBZKyd3yIQc]https://www.youtube.com/watch?v=QBZKyd3yIQc[/video] To post audio/mp3: [noparse][mp3]http://www.youtubetomp3.com/load/YouTube/4By8YHOnILo/e234e9356c94a96373e058469407110e/[/mp3][/noparse] Result: [mp3]http://www.youtubetomp3.com/load/YouTube/4By8YHOnILo/e234e9356c94a96373e058469407110e/[/mp3]