code: public function handle(ModelToLabelEvent $event)
      {
          if (!$this->wantToHandle($event)) {
              return;
          }
  
          if ($event->getModel()->getProperty('isdefault')) {
              $event->setLabel(
                  sprintf(
                      '%s <span style="color:#b3b3b3; padding-left:3px">[%s]</span>',
                      $event->getLabel(),
                      $this->translator->trans('MSC.fallback', [], 'contao_default')
                  )
              );
          }
      }
docs: Render the html for the input screen condition.
  
  @param ModelToLabelEvent $event The event.
  
  @return void