code: public function render(ElementInterface $element, $buttonContent = null)
      {
          $openTag = $this->openTag($element);
  
          if (null === $buttonContent) {
              $buttonContent = $element->getLabel();
              if (null === $buttonContent) {
                  throw new Exception\DomainException(
                      sprintf(
                          '%s expects either button content as the second argument, ' .
                          'or that the element provided has a label value; neither found',
                          __METHOD__
                      )
                  );
              }
  
              if (null !== ($translator = $this->getTranslator())) {
                  $buttonContent = $translator->translate(
                      $buttonContent,
                      $this->getTranslatorTextDomain()
                  );
              }
              $element->setLabel('');
          }
  
          $escape = $this->getEscapeHtmlHelper();
          $translator = $this->getTranslator();
          $buttonContent = '<div><div class="processing yk-hidden"><span class="fa-spin yk-icon-spinner yk-icon"></span> ' . $translator->translate('processing', $this->getTranslatorTextDomain()) . '</div><div class="default">' . $escape($buttonContent) . '</div></div>';
  
          return $openTag . $buttonContent . $this->closeTag();
      }
docs: Render a form <input> element from the provided $element
  
  @param  ElementInterface $element
  @throws Exception\DomainException
  @return string