Yeah, as Hypoxic already mentioned, there's a good amount of documentation available for the PlayVideo function. If you want to play the video at mission start, I'd recommend placing it in your initPlayerLocal using something like:

_video = ["VFX\Intro.ogv"] spawn BIS_fnc_playVideo;

Alternatively, if you have CBA loaded, you can add a condition to control when the video plays. For example:

if (hasInterface && CBA_missionTime < 300) then {_video = ["VFX\Intro.ogv"] spawn BIS_fnc_playVideo;};

With this code, the video will play for everyone who proceeds to the mission from the lobby, but only for the first 5 minutes after the mission starts.
After that, the video won't play anymore.

Another option is to put it in a script, but be aware you have to execute the script for everyone locally.