That's defined in Edit or delete a forum > Choose Forum > Allow HTML (Use caution with this. Allowing HTML allows your users to post things like javascript which could lead to vulnerabilities) Here you can choose whether to add it on a per forum basis.
Beware, this is a pretty big security risk to leave on. Not only can someone post malicious content that could hurt your site but it will also give them the ability to add popups, etc. to their posts.
