Does anyone know how to make videos autoplay when the thread is opened?

I saw it before in a few shop threads (they used a song in their case), and I would like to know to use it.

Could you find me a link? I have a suspicion and if they're doing what I think then it may have to be looked into.

There used to be threads, but I couldn't find them.

Anyway I already got my answer from someone else.