You will get more joy and it will be more secure if you make your own end point in a module, just for registering users.
First, in your module's .module file, make use of the hook_menu function
/*
* hook_menu
*/
function yourmodule_menu() {
$items['yourmodule/user/register'] = array(
'page callback' => 'yourmodule_user_register',
'access arguments' => array('access content'),
);
return $items;
}
Then create the callback function:
/*
* hook_menu page callback
*/
function yourmodule_user_register() {
// if you want to send the payload using content type 'application/json'
$json = file_get_contents('php://input');
$obj = json_decode($json);
// otherwise use $_POST
// if you have autoloader set up
$register = new YourmoduleUserRegister($obj);
// otherwise add in your logic here to validate and save user data
}
Once you have validated your user data, using the drupal API methods user_load_by_mail() and user_load_by_name(), use user_save() to register the user. You can whitelist origin URLs and do other security things here too.