When you are at youtube, dont just copy what is in the address bar. right underneath the video is a little box to the right of it that says embed. click that. then customize your options. then copy the embed code.
next come on here and click reply, the "go advanced" you will see a little youtube box next to the picture and link adding buttons. just click that and post into the [Y T][/Y T] squares
It would look like this....