1) Go to the video's YouTube page.
2) Copy the url from the place where you type in addresses on your browser.
3) Paste into a Crappity post.
That's it.
Things that don't work:
1) Those youtu.be links that YouTube gives you to share.
2) The link from a Google results page showing the video.
3) The link from the video poster's member page.
This technique also works for Vimeo and a bunch of other small sites. Try your luck and hit "preview."
Alternate/Advanced Method (Also Works on other, non-YouTube kinds of videos you want to embed)
1) Copy the "embed" code.
2) Start a Crappity post, put in these tags:
[html][/html]
3) Paste the embed code between those two tags.
4) Post.
5) If your post doesn't have anything except the video, you may get an error message saying it's an empty post. Just add a period. That works fine.