how to a link in Admin black toolbar
/**
    * Implements hook_toolbar().
    */
    function backlinks_notifications_toolbar() {
        $items = [];
        $items['toolbar_menu_backlinks_notifications'] = [
            '#type' =&gt; 'toolbar_item',
            'tab' =&gt; [
                '#type' =&gt; 'link',
                '#title' =&gt; 'Notifications',
                '#url' =&gt;  Url::fromRoute('&lt;front&gt;'),
                '#options' =&gt; [
                    'attributes' =&gt; array(
                        'title' =&gt; t('Backlinks Notification'),
                        'class' =&gt; array(
                        'toolbar-item',
                        'toolbar-icon',
                        'toolbar-icon-backlinks-notifications',
                        ),
                    ),
                ],
            ],
            '#weight' =&gt; 999,
            '#attached' =&gt; array(
                'library' =&gt; array(
                    'backlinks_notifications/backlinks_notifications',
                ),
            ),
        ];
        return $items;
    }


Hook available at 
https://api.drupal.org/api/drupal/core%21modules%21toolbar%21toolbar.api.php/function/hook_toolbar/8.2.x