Im so sory, im find this in flarum/subscribe.
Maybe it help, if anyone else will do this.
use Flarum\Event\ConfigureUserPreferences;
$events->listen(ConfigureUserPreferences::class, function (ConfigureUserPreferences $event) {
$event->add('AnyPreference', 'boolval', false);
});
And name for preference is notify_{notyfy type}_{notify method}, for example: notify_userSuspended_alert.
