code: public function addExtra($id, IFormField $formField)
      {
          $formField->setId($id);
  
          return $this->addFormField($formField, true);
      }
docs: Add an extra field to the form.
  
  An extra field is not tracked or mapped to the subject.
  
  @param string     $id
  @param IFormField $formField
  
  @return IFormField