cakephp/tests/test_app/TestApp/Form/AppForm.php

<?php
/**
 * CakePHP(tm) : Rapid Development Framework (https://cakephp.org)
 * Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
 *
 * Licensed under The MIT License
 * For full copyright and license information, please see the LICENSE.txt
 * Redistributions of files must retain the above copyright notice.
 *
 * @copyright     Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
 * @link          https://cakephp.org CakePHP(tm) Project
 * @since         3.5.13
 * @license       https://opensource.org/licenses/mit-license.php MIT License
 */
namespace TestApp\Form;

use Cake\Form\Form;

class AppForm extends Form
{
    protected $_schemaClass = FormSchema::class;
}