code: public void setCustomRequestParameter(String param, String... values) {
          _customRequestParameters.putAll(param, Arrays.asList(values));
      }
docs: Sets custom request parameters.  Custom parameters may include new features not yet officially supported or
  additional parameters to existing calls not intended for widespread use.  As such this method is not typically
  used by most clients.  Furthermore, adding additional parameters may cause the request to fail.