code: public function onGeneratePage(PageModel $pageModel, LayoutModel $layoutModel): void
      {
          $calendarfeeds = StringUtil::deserialize($layoutModel->calendarfeeds);
  
          if (empty($calendarfeeds) || !\is_array($calendarfeeds)) {
              return;
          }
  
          $this->framework->initialize();
  
          /** @var CalendarFeedModel $adapter */
          $adapter = $this->framework->getAdapter(CalendarFeedModel::class);
  
          if (!($feeds = $adapter->findByIds($calendarfeeds)) instanceof Collection) {
              return;
          }
  
          /** @var Template $template */
          $template = $this->framework->getAdapter(Template::class);
  
          /** @var Environment $environment */
          $environment = $this->framework->getAdapter(Environment::class);
  
          foreach ($feeds as $feed) {
              $GLOBALS['TL_HEAD'][] = $template->generateFeedTag(
                  sprintf('%sshare/%s.xml', ($feed->feedBase ?: $environment->get('base')), $feed->alias),
                  $feed->format,
                  $feed->title
              );
          }
      }
docs: Adds the feeds to the page header.