Test video:

Yep, seems to be working. With a YouTube video, easiest way is to click the Share button, then the Embed option, which will give you HTML embed code. Copy and paste it to the LttD post, and make absolutely sure to enable HTML in your post. This post was written using both HTML and UBBCode; if you use only HTML, be aware that you'll have to use the br command to make line breaks, and usual forum markup won't work.
"The devil I'll bring you," answered Hagen. "I have enough to carry with my shield and breastplate; my helm is bright, the sword is in my hand, therefore I bring you naught."