code: def update_custom_field(self, custom_field_key, field_name,
                              visible_in_preference_center):
          
          custom_field_key = quote(custom_field_key, '')
          body = {
              "FieldName": field_name,
              "VisibleInPreferenceCenter": visible_in_preference_center}
          response = self._put(self.uri_for("customfields/%s" %
                                            custom_field_key), json.dumps(body))
          return json_to_py(response)
docs: Updates a custom field belonging to this list.