edlp_d8/core/modules/contact/tests/modules/contact_storage_test/contact_storage_test.module

<?php

/**
 * @file
 * Contains custom contact message functionality for ContactStorageTest.
 */

use Drupal\contact\ContactFormInterface;
use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_entity_base_field_info().
 */
function contact_storage_test_entity_base_field_info(EntityTypeInterface $entity_type) {
  if ($entity_type->id() == 'contact_message') {
    $fields = [];

    $fields['id'] = BaseFieldDefinition::create('integer')
      ->setLabel(t('Message ID'))
      ->setDescription(t('The message ID.'))
      ->setReadOnly(TRUE)
      ->setSetting('unsigned', TRUE);

    return $fields;
  }
}


/**
 * Implements hook_entity_type_alter().
 */
function contact_storage_test_entity_type_alter(array &$entity_types) {
  /** @var $entity_types \Drupal\Core\Entity\EntityTypeInterface[] */
  // Set the controller class for nodes to an alternate implementation of the
  // Drupal\Core\Entity\EntityStorageInterface interface.
  $entity_types['contact_message']->setStorageClass('\Drupal\Core\Entity\Sql\SqlContentEntityStorage');
  $keys = $entity_types['contact_message']->getKeys();
  $keys['id'] = 'id';
  $entity_types['contact_message']->set('entity_keys', $keys);
  $entity_types['contact_message']->set('base_table', 'contact_message');
}

/**
 * Implements hook_form_FORM_ID_alter() for contact_form_form().
 */
function contact_storage_test_form_contact_form_form_alter(&$form, FormStateInterface $form_state) {
  /** @var \Drupal\contact\ContactFormInterface $contact_form */
  $contact_form = $form_state->getFormObject()->getEntity();
  $form['send_a_pony'] = [
    '#type' => 'checkbox',
    '#title' => t('Send submitters a voucher for a free pony.'),
    '#description' => t('Enable to send an additional email with a free pony voucher to anyone who submits the form.'),
    '#default_value' => $contact_form->getThirdPartySetting('contact_storage_test', 'send_a_pony', FALSE),
  ];
  $form['#entity_builders'][] = 'contact_storage_test_contact_form_form_builder';
}
/**
 * Entity builder for the contact form edit form with third party options.
 *
 * @see contact_storage_test_form_contact_form_edit_form_alter()
 */
function contact_storage_test_contact_form_form_builder($entity_type, ContactFormInterface $contact_form, &$form, FormStateInterface $form_state) {
  $contact_form->setThirdPartySetting('contact_storage_test', 'send_a_pony', $form_state->getValue('send_a_pony'));
}