if you mean here? i think you need to use the old, long-form URL/link, and not the new abbreviated ones they use now.
select old/long form link (whatever they call it)
go back to BTD forum
click on the blue box (no, not *that* blue box. i mean the 5th square over from the left for "enter media tag")
select YouTube video
cntl+v or right click and paste
or... at least that's how i do it