I know this is old but I needed it to, so here is the answer for anyone else finding this on google:
1) Open bb-includes/functions.bb-meta.php
2) Find this line:
case 'url':
$option = 'uri';
3) After it, add:
case 'uri':
return "http://".$_SERVER['HTTP_HOST']."/forum/";
break;
Replace the “/forum/” part with the proper URL to your forums. For example
http://blah.com/bbpress/
would be:
return "http://".$_SERVER['HTTP_HOST']."/bbpress/";
