1. Create a webservice custom plugin, that extend core_user_external . Example: // Example: local/mywebservices/version.php defined('MOODLE_INTERNAL') || die(); $plugin->version = 2020022416; $plugin->requires = 2018051709; $plugin->component = 'local_mywebservices'; $plugin->maturity = MATURITY_BETA;     // Example: local/mywebservices/db/services.php $functions = array( 'local_mywebservices_create_users' => array( 'classname' => 'local_mywebservices_external', 'methodname' => 'create_users', 'classpath' => 'local/mywebservices/class/external.php', 'description' => 'Create users (including phone1).', 'type' => 'write', 'capabilities' => 'moodle/user:create' ) );     // Example: local/mywebservices/class/external.php class local_mywebservices_external extends core_user_external { public static function create_users_parameters() { $parameters = parent::create_users_parameters(); $parameters->keys['users']->content->keys['phone1'] = new external_value( core_user::get_property_type('phone1'), 'The phone number of the user', VALUE_OPTIONAL); return $parameters; } } 2. Enable webservices and create a webservice user and token in the administration, to allow the use of the custom webservice. ( https://docs.moodle.org/38/en/Using_web_services ). The user related to the token must have the capabilities to create and update users, as we want to extend the webservices core_user_create_users and core_user_update_users. 3. Call your custom webservice. Example: POST /webservice/rest/server.php?moodlewsrestformat=json&wstoken=TOKEN&wsfunction=local_mywebservices_create_users HTTP/1.1 Host: mdl-local.docker.test Content-Type: application/x-www-form-urlencoded Authorization: Basic d3M6d3M=   users%5B0%5D%5Busername%5D=test-username&users%5B0%5D%5Bpassword%5D=test-password-123&users%5B0%5D%5Bfirstname%5D=test-firstame&users%5B0%5D%5Blastname%5D=test-lastname&users%5B0%5D%5Bemail%5D=test-email%40example.com&users%5B0%5D%5Bphone1%5D=0123456789 As for the webservices we are extending, the request must be POST. Any post webform and application to forge POST request can do the job. I personally like postman to do testing. Without the correction: Notice that the webservice returns an "invalid parameters error" With the correction Notice that new optional field (phone1) of the new created user is filled. Same process for update.