You can't in this version.  The workaround I use is:
Since websites generally use only one of <http://domain.com>
or <http://www.domain.com> ,but will accept referers from
either, I set up my own http server at the unused one of the
above (I add an entry to my HOSTS file to prevent DNS from
going to  the actual site) with a html file pointing to the
actual site.  this will effectively set the referer