Customize the WordPress Admin Toolbar: Why and How to Do It

// add a parent item to the WordPress admin toolbar

function add_link_to_admin_bar($admin_bar) {
    $args = array(
        'id' => 'my-custom-link',
        'title' => 'My Custom Link', 
        'href' => 'https://www.mydomain.com'
    );
    $admin_bar->add_node($args);
}
	
add_action('admin_bar_menu', 'add_link_to_admin_bar', 999);