This is a simple (and somewhat silly) example showing how to use AuthManager to add extra fields to the registration form.

1. Define the fields with an authentication request object.

   <?php
   
   namespace MediaWiki\Extension\MyExt\Auth;
   
   use MediaWiki\Auth\AuthenticationRequest;
   
   class MyExtraFieldsAuthenticationRequest extends AuthenticationRequest {
       // Mark this AuthenticationRequest as optional.
       public $required = self::OPTIONAL;
   
       /** @var string */
       public $eyeColor;
   
       /** @var string */
       public $favoriteFood;
   
       public function getFieldInfo() {
           return [
               'eyeColor' => [
                   'type' => 'string',
                   'label' => wfMessage( 'eyecolor-label' ),
               ],
               'favoriteFood' => [
                   'type' => 'string',
                   'label' => wfMessage( 'favoritefood-label' ),
                   'help' => wfMessage( 'favoritefood-help' ),
                   // Marking the field as optional means the AuthenticationRequest can
                   // be created if it's not present. Marking the AuthenticationRequest
                   // as optional means authentication can proceed if it was not created.
                   'optional' => true,
               ],
           ];
       }
   }
   

2. Add a secondary authentication provider (a handler that is called when the User object representing the user becomes available) that signals the need for the request, and processes it:

   <?php
   
   namespace MediaWiki\Extension\MyExt\Auth;
   
   use MediaWiki\Auth\AuthManager;
   use MediaWiki\Auth\AbstractSecondaryAuthenticationProvider;
   use MediaWiki\Auth\AuthenticationRequest;
   use MediaWiki\Auth\AuthenticationResponse;
   use MediaWiki\Extension\MyExt\Auth\MyExtraFieldsAuthenticationRequest;
   use MediaWiki\Extension\MyExt\Auth\MyExtraFieldsHandler;
   
   class MyExtraFieldsSecondaryAuthenticationProvider
       extends AbstractSecondaryAuthenticationProvider
   {
       public function getAuthenticationRequests( $action, array $options ) {
           if ( $action === AuthManager::ACTION_CREATE ) {
               return [ new MyExtraFieldsAuthenticationRequest() ];
           }
           return [];
       }
   
       public function beginSecondaryAuthentication( $user, array $reqs ) {
           return AuthenticationResponse::newAbstain();
       }
   
       public function beginSecondaryAccountCreation( $user, $creator, array $reqs ) {
           $req = AuthenticationRequest::getRequestByClass( $reqs,
               MyExtraFieldsAuthenticationRequest::class );
           if ( !$req ) {
               // fields were left empty
               return AuthenticationResponse::newPass();
           }
   
           if ( $req->eyeColor === 'pink' ) {
               return AuthenticationResponse::newFail(
                   wfMessage( 'error-wrong-eyecolor', $req->eyeColor ) );
           }
   
           // At this point, the user has been written into the database; $user->getId() is safe to use.
           MyExtraFieldsHandler::saveExtraFields( $user, $req->eyeColor, $req->favoriteFood );
   
           return AuthenticationResponse::newPass();
       }
   }
   

3. (Optional) Add a hook for customizing the presentation of the fields (see HTMLForm for the syntax of $formDescriptor):

   <?php
   
   namespace MediaWiki\Extension\MyExt;
   
   use MediaWiki\SpecialPage\Hook\AuthChangeFormFieldsHook;
   use MediaWiki\Auth\AuthenticationRequest;
   
   class MyExtraFieldsHooks implements AuthChangeFormFieldsHook {
       /**
        * @param AuthenticationRequest[] $requests
   	 * @param array $fieldInfo
   	 * @param array &$formDescriptor
   	 * @param string $action
        */
       public function onAuthChangeFormFields( $requests, $fieldInfo, &$formDescriptor, $action ) {
           if ( isset( $formDescriptor['eyeColor'] ) ) {
               $formDescriptor['eyeColor']['cssclass'] = 'eye-color';
           }
       }
   }
   

4. Register the provider (and the hook if it was added):

       ...
       "AuthManagerAutoConfig": {
           "secondaryauth": {
               "MyExtraFieldsSecondaryAuthenticationProvider": {
                   "class": "MediaWiki\\Extension\\MyExt\\Auth\\MyExtraFieldsSecondaryAuthenticationProvider",
                   "sort": 0
                }
           }
       },
       "Hooks": {
           ...
           "AuthChangeFormFields": "main"
       },
       "HookHandlers": {
           "main": {
               "class": "MediaWiki\\Extension\\MyExt\\Auth\\MyExtraFieldsSecondaryAuthenticationProvider"
           }
       }
       ...