Please always include the URL to your forum.
If you mean you want the chat box to be in a post box, it won't work well.
In a post, html will only show with the result intended if a member is logged in, AND purposely set their profile to allow HTML.
You would have to add the html elements such as "embed" in the list of "allowed HTML" in the admin panel. But as I said, many people going to your forum won't see the chat box in a post. They will just see the code for it.