code: public function getAttributes()
      {
          $attributes = array(
              'placeholder' => $this->config['defaultparts']['scheme'] . "://example.com" //example url
          );
          if ($this->config['html5validation']) {
              $attributes += array(
                  'type' => 'url', //html5 field type
                  'pattern' => 'https?://.+', //valid urls only
              );
          }
  
          return array_merge(
              parent::getAttributes(),
              $attributes
          );
      }
docs: Set additional attributes
  @return array Attributes