too long

I'm trying to add new users to moodle 3.2 using a REST web service, and i want to customize thees fields (phone1, department, institution) in the student profile. I used this code

$token = 'a38805c00f33023f7854d5adc720c7a7';
$domainname = 'http://localhost/moodle';
$functionname = 'core_user_create_users';
$restformat = 'json'; 

$user2 = new stdClass();
$user2->username = strtolower( $rsnew['Serial']);
$user2->password = $rsnew['pass'];
$user2->firstname = $rsnew['Fname'];
$user2->lastname = $rsnew['Lname'];
$user2->email = $rsnew['Email'];
$user2->lang = 'en';
$user2->auth = 'manual';
$user2->country = $rsnew['Country'];
$user2->timezone = '99';
$user2->phone1 = $rsnew['phone'];
$user2->department = $rsnew['dept'];
$user2->institution = $rsnew['branch'];
$user2->idnumber = $rsnew['grade'];
$users = array($user2);
$params = array('users' => $users);
$serverurl = $domainname . '/webservice/rest/server.php'. '?wstoken=' . $token . '&wsfunction='.$functionname;
require_once('./curl.php');
$curl = new curl;

$restformat = ($restformat == 'json')?'&moodlewsrestformat=' . $restformat:'';
$resp = $curl->post($serverurl . $restformat, $params);

But i get this error :

{
    "exception": "invalid_parameter_exception",
    "errorcode": "invalidparameter",
    "message": "Invalid parameter value detected",
    "debuginfo": "users => Invalid parameter value detected: Unexpected keys (phone1, department, institution) detected in parameter array."
}

What should I do to fix that?

Just as the error suggests, that web service does not support the fields you are giving it. You can refer to the function itself to find out what fields are supported and what data they must contain.

core_user_get_users

I am not aware of a workaround for your problem using existing web services. However, you can create your own which includes those additional fields.

Note that this sounds to me like this is a desirable feature and should be raised on the issue tracker.