you don't , apparently your server is setup to not use the www
which is great, because it is no longer needed for urls
the www is a hold over from years ago when it was needed to tell the difference between machines. some were web servers only ( www )
some were ftp only ( ftp. ) some were email only, etc. etc.
nowdays, everthing is often on one machine, therefore, no need for the www.
it was always just an alias.
your host has their server setup to not use it and to redirect any www calls back to your true domain name which is just domain.com not www.domain.com
so, forget about it, don't use it in your admin panel configuration, don't use it in any links to your board, etc.
if anyone mistakenly uses it to access your board, it will be rediredted to your true domain name anyway.
also, never put the http:// in for your domain name in the admin panel.
first it won't work, becuase the script already prepends that to all the links so you would wind up trying to go to http://http://yourdomain.com
is not a domain name, it is a url
domain.com is a domain name.