/**
* Renders hidden form fields for referrer information about
* the current request.
*
* @return string Hidden fields with referrer information
*/
protected function renderHiddenReferrerFields()
{
$tagBuilder = new TagBuilder('input');
$tagBuilder->addAttribute('type', 'hidden');
$tagBuilder->addAttribute('name', $this->prefixFieldName('__state'));
$serializedFormState = base64_encode(serialize($this->arguments['object']->getFormState()));
$tagBuilder->addAttribute('value', $this->hashService->appendHmac($serializedFormState));
return $tagBuilder->render();
}