Add User - "Bad Request - You haven't filled in some mandatory fields"

I’m new to the Docebo API and I am trying to get the API to add a user.

I can add a user via the “API Explorer” using these fields:

{
  "userid": "killer@domain.com",
  "email": "killer@domain.com",
  "password": "some_random_stuff",
  "firstname": "Killer",
  "lastname": "Cat",
  "force_change": 1,
  "level": 6,
  "send_notification_email": true,
  "select_orgchart": {
    "375": 1
  }
}

I’m using php to connect to the API, and submitting these fields there via the following construct:

$data["userid"] = "killer@domain.com";
$data["email"] =  "killer@domain.com";
$data["password"] =  "some_random_stuff";
$data["firstname"] = "Killer";
$data["lastname"] = "Cat";
$data["force_change"] = 1;
$data["level"] = 6;
$data["send_notification_email"] = true;
$data["select_orgchart"] = array("375" => 1);

// Build a POST request
$opts = array(
    'http' => array(
        'method' => 'POST',
        'header' => array(
            'Authorization: Bearer '.$token,
            'Content-Type: application/x-www-form-urlencoded\r\n',
        ),
        'content' => http_build_query($data),
        'ignore_errors' => true
    )
);
$context = stream_context_create($opts);

// Poll the API
$get_response = file_get_contents($url, false, $context);

The URL I am using is "/manage/v1/user".

I get this  response from the php request:

{

"name":"Bad Request",

"message":[

    {"field":"userid","message":"You haven't filled in some mandatory fields"},

    {"field":"password","message":"You haven't filled in some mandatory fields"}

],

"code":1002,

"status":400

}

This suggests that there are two fields missing… but clearly I have filled in both the userid and password fields.

FWIW - I get the same response if I use php curl to run the POST request.

Any ideas what I might be doing wrong?

Any and all suggestions / help would be most welcome.

Thanks!