It only works in the picture forum and I've noticed it won't show up in the preview post (just shows a big empty space) in the picture forum but if you post it, refresh the page it's there and working.
I think you can only do it in the Pictures section.
But for youtube videos, you just click "share" then "embed". Copy the code and paste it into your reply. It won't show up at first, but if you refresh the page after you post it, it will.