It is just for security that no one knows that there is a forum

I will try the above solution, thanks in advance!
I tried the solution given. Unfortunately, it prevented the SMF Stylesheet from being read. I'm not sure why.
In the mean time, I have used the following workaround. In the /smf/index.php, i have added the following lines at the top.
/*
* Get the URI path that was typed in the Address bar
* and strip the filename so we're left with the path
*/
$URI = $_SERVER["REQUEST_URI"];
$URI = substr( $URI, 0, strrpos( $URI, "/" ));
/*
* If the path does not match the path for the Joomla
* installation, redirect to the correct location
* for the Forum under Joomla.
* NOTE: Itemid may be different on some installations.
*/
if ( $URI != "/go/portal" ) {
header ("Location: http://www.domain.com/go/portal/index.php?option=com_smf&Itemid=51");
}