Okay... took some trial and error but to embed a youtube vid here on the forum you have to go to the video you want to post on youtube. Underneath the vid will be a tab that says "Share". Click that and you'll get some more options including "Embed" but Embed does NOT work here for some reason. Want you want to use is the link generated by default under the first option labelled "Share This Video". It looks similar to the url of the video but it is slightly different.
After you copy that code come here and select the Full Version comment box (the Quick Reply comment box does not have the proper code generator). Then when in the Full Version comment box look in the upper right hand corner. You will see two buttons. One for images and the one on the right which has further options (if you hover over it it will say Specail PGD Code). Click that second one and the first option is to generate a code to embed youtube vids. Click it and the code will pop into the box. Now paste the Share code you snagged from youtube in between the two tags.
Now your vid should be embedded. You can also simply type the PGD Code which may or may not work in the Quick Reply box. I'll let you play with that if you are so inclined.