On a Macbook Air . . .
Go to the relevant YouTube page.
Highlight the URL in your web browser's address bar..
Press Command and C together.
Come to an AnswerBank post.
On a fresh line in the post, press Command and V together.
(Don't type anything else on the same line).
You'll then see the YouTube URL in your post.
Submit your post as usual (by clicking the 'Answer Now' button).
When your post appears on the site, your YouTube video will then be embedded into it.