There are a bunch of symbols above your "reply" box, the second row has three symbols (a generic "picture" symbol on the left, a speech bubble with an "S" on the right), the middle symbol with the easel is the one that you're looking for. Simply open a new tab/window in your browser, go to the Youtube video you want to post, copy its URL from the address bar, and then click on the aforementioned "easel" symbol. You'll be prompted to enter an URL, so paste the URL from your Youtube video and click on "Embed".
That's all there is to it!
And yea, Crüxshadows have been around for quite a while now, haven't they?
Or you could copy the URL from the address bar and paste it to your post....and skip all that other crap.