hook_toolbar

function toolbar_test_toolbar() {
  $items = [];

  $items['scribe_posts'] = array(
    '#type' => 'toolbar_item',
    'tab' => array(
      '#type' => 'link',
      '#title' => t('Posts'),
      '#url' => Url::fromUri('http://www.google.com'),
      '#attributes' => array(
        'class' => array('toolbar-icon', 'toolbar-icon-system-admin-content'),
      ),
    ),
  );

  return $items;
}