You could use a PHP Script to redirect the Site:
header( 'Location: http://IPAdress' ) ;
That redirection will replace the Browser's URL address from "http://www.domainname.com/forum/" to "http://18.104.22.168" - It works fine but it doesn't look good as your visitor will see that IP address instead of the sub-folder address
The Single HTML Frame method also works fine, but the URL will stay remain "http://www.domainname.com/forum/" even you browse to other pages/different path on that forum site.
The best solution is to use sub-domain as mentioned by others above.