Task-13 Add a field to an existing form

lotus.module

<?php
/**
 * @file
 * Add a field to an existing form.
 */

use \Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function lotus_form_user_register_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Add a checkbox to registration form for terms.
  $form['terms'] = array(
    '#type' => 'checkbox',
    '#title' => t("I accept the website's terms."),
    '#required' => TRUE,
  );
}