The first thing you need to do is get Sendy working with that module. That will make sure all the lists are setup properly.
Then, if you want to add a user to a list when they are registering through another extension, you need to call the following PHP code:
$sendy = Sendy::getInstance();
$return = $sendy->subscribeUser($email, $listId);
I hope that helps, but if you're looking for something else, please let me know.
Thanks,
Alex
How to add Sendy integration to any extension
