Adjust Lines 4-6 as desired.
function custom_link() {
global $wp_admin_bar;
$wp_admin_bar->add_menu( array(
'id' => 'custom-link',
'title' => 'Name of your custom link',
'href' => 'https://domain.com/customlink',
) );
}
add_action( 'wp_before_admin_bar_render', 'custom_link' );