Flarum\Formatter\Formatter::configureExternalLinks PHP Метод

    protected function configureExternalLinks(Configurator $configurator)
    {
        $dom = $configurator->tags['URL']->template->asDOM();
        foreach ($dom->getElementsByTagName('a') as $a) {
            $a->setAttribute('target', '_blank');
            $a->setAttribute('rel', 'nofollow');
        }
        $dom->saveChanges();
    }