It's not supported; HTML only.
A suggestion has been made for it.
Can I use HTML in posts?
Currently it is only possible to use BB Code in posts, HTML is not supported.
How do I stop BB Code from being parsed in posts?
Use the plain tags like so:
This content will [b]not[/b] be parsed.
Can I use BB Code in Pages?
Currently it is only possible to use HTML in Pages, BB Code is not supported.
http://xenforo.com/community/threads/frequently-asked-questions.5183/#post-180445