docs: def get_add_link(self):
          """
          Appends the popup=1 query string to the url so the
          destination url treats it as a popup.
          """
  
          url = super(TaggedRelationWidget, self).get_add_link()
          if url:
              qs = self.get_add_qs()
              if qs:
                  url = "%s&%s" % (url, urllib.urlencode(qs))
          return url