/**
 * Implements hook_menu_local_tasks_alter().
 */
function {{ machine_name }}_menu_local_tasks_alter(&$data, $route_name, \Drupal\Core\Cache\RefinableCacheableDependencyInterface &$cacheability) {

  // Add a tab linking to node/add to all pages.
  $data['tabs'][0]['node.add_page'] = [
      '#theme' => 'menu_local_task',
      '#link' => [
          'title' => t('Example tab'),
          'url' => Url::fromRoute('node.add_page'),
          'localized_options' => [
              'attributes' => [
                  'title' => t('Add content'),
              ],
          ],
      ],
  ];
  // The tab we're adding is dependent on a user's access to add content.
  $cacheability->addCacheTags(['user.permissions']);
}