Using REST to create user with CAS username I would like my CAS server to POST an account to my client site when a new user registers. My CAS server is using D7, clients are using D8.

Using the entity/user endpoint I can create a new user, assign roles and  complete custom fields. It all works well apart from adding the cas_username.

When trying to pass the CAS username I get en error:


  "InvalidArgumentException: Field cas_username is unknown. in Drupal\Core\Entity\ContentEntityBase->getTranslatedField() (line 586 of /mysite/html/core/lib/Drupal/Core/Entity/ContentEntityBase.php)."


Is it possible to POST a user with cas_username field via the create user endpoint? 

Do I need to create a custom endpoint that extends the functions of the default entity/user endpoint?
Answering with the workaround I used. It is far from ideal, but works. It relies on having access to the DB of the client site. This uses D7 rules with the php-filter module. It would be much more elegant in it's own custom module.

The site sending the POST request uses sql to update the authname table on the site receiving the POST request. The created user then has a cas-username.

Below is the code for the rule action - creates the POST request, then alters the DB.

// Generate random password
$bytes = random_bytes(10);
$pass = bin2hex($bytes);

// data for user fields
$data = array(
  'name' =&gt; array(
    'value' =&gt; '[account:name]'
    ),
 'mail' =&gt; array(
   'value' =&gt; '[account:mail]'
 ),
 'status' =&gt; array(
   'value' =&gt; 1
  ),
 'pass' =&gt; array(
   'value' =&gt; $pass
    )
  );

// POST code
$data_string = json_encode($data);

// POST with cURL
$url = 'https://example.com/restendpoint';

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_USERPWD, "username:password");
curl_setopt($ch, CURLOPT_HTTPAUTH, CURLAUTH_BASIC);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));

$response = curl_exec($ch);

// Get UID from response
$newuser = json_decode($response, true);
$newuseruid = $newuser['uid'][0]['value'];

// DB update
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "dbname";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn-&gt;connect_error) {
    die("Connection failed: " . $conn-&gt;connect_error);
} 

$sql = "INSERT INTO authmap (uid, provider, authname, data)
VALUES ($newuseruid, 'cas', '[account:name]', 'N;')";

if ($conn-&gt;query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "&lt;br&gt;" . $conn-&gt;error;
}
$conn-&gt;close();

curl_close($ch);