public static function add_filter_to_form(\MoodleQuickForm &$mform) {
        $options = ['multiple' => true];

        $filtername = self::get_filter_name();
        $key = "filter_{$filtername}";

        $mform->addElement('course', $key, get_string($key, 'tool_usertours'), $options);
        $mform->setDefault($key, '0');
        $mform->addHelpButton($key, $key, 'tool_usertours');
    }