Rich text editor in a custom Drupal form. Also see A custom Drupal form with multiple file upload. Also see https://www.lucius.digital/en/blog/rich-text-editor-custom-drupal-form-example-code-including-data-sanitization-security

MessageForm.php

<?php

namespace Drupal\ol_messages\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Component\Utility\Html;
use Drupal\ol_messages\Services\OlMessages;
use Symfony\Component\DependencyInjection\ContainerInterface;


/**
 * Class MessageForm.
 */
class MessageForm extends FormBase {

  /**
   * @var $messages
   */
  protected $messages;


  /**
   * Class constructor.
   */
  public function __construct(OlMessages $messages) {
    $this->messages = $messages;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('olmessages.messages'),
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'message_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {

    $id = null;
    // For 'edit' mode: do a check here to fill $id.

    // Build form.
    $form['message_id'] = [
      '#type' => 'hidden',
      '#default_value' => $id,
      '#weight' => '0',
    ];
    $form['body'] = [
      '#type' => 'text_format',
      '#format' => 'basic_html',
      '#weight' => '20',
      '#required' => true,
    ];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {

    if (strlen($form_state->getValue('body')['value']) <= 3) {
      // Set an error for the form element with a key of "title".
      $form_state->setErrorByName('body', $this->t('A message should contain more than 3 characters.'));
    }
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {

    // Get data.
    $id = Html::escape($form_state->getValue('message_id'));
    $body = $form_state->getValue('body')['value'];
    $body = check_markup($body,'basic_html');

    // Existing, update message.
    if(is_numeric($id)){
      $this->messages->updateMessage($id, $body);
    }
    // New, save message.
    elseif(empty($id)){
      $this->messages->saveMessage($body);
    }
  }

}