CustomerForm.php

<?php

namespace Drupal\hosting_management\Form;

use Drupal\Core\Entity\ContentEntityForm;
use Drupal\Core\Form\FormStateInterface;
use Drupal\message\Entity\Message;

/**
 * Form controller for Customer edit forms.
 *
 * @ingroup hosting_management
 */
class CustomerForm extends ContentEntityForm {

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    /* @var $entity \Drupal\hosting_management\Entity\Customer */
    $form = parent::buildForm($form, $form_state);

    // $noteform = \Drupal::formBuilder()->getForm('Drupal\hosting_management\Form\CustomerNoteForm');
    // unset($noteform['#theme_wrappers']);

    $noteform = array();
    $noteform['#prefix'] = '<div id="note-form">';
    $noteform['#suffix'] = '</div>';
    $noteform['note'] = array(
      '#type' => 'textarea',
      '#title' => $this->t('Note'),
      '#weight' => 1,
    );
    $noteform['actions']['#type'] = 'actions';
    $noteform['actions']['#weight'] = 2;
    $noteform['actions']['submit'] = array(
      '#type' => 'submit',
      '#value' => $this->t('Add Note'),
      '#name' => 'note-submit',
      '#button_type' => 'primary',
      '#callback' => '::customer_note_form_submit',
      '#ajax' => array(
        'callback' => array($this, 'customer_note_form_js_submit'),
        'wrapper' => 'note-form',
        'method' => 'replace',
        'effect' => 'fade',
      ),
    );

    $noteform['customer'] = array('#type' => 'hidden', '#value' => $form_state->getFormObject()->getEntity()->id());
    $noteform['note_list'] = views_embed_view('customer_messages', 'default', $form_state->getFormObject()->getEntity()->id());
    $noteform['note_list']['#weight'] = 3;

    $form['note_form'] = $noteform;

    $entity = $this->entity;

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function save(array $form, FormStateInterface $form_state) {
    $entity = &$this->entity;

    $status = parent::save($form, $form_state);

    switch ($status) {
      case SAVED_NEW:
        drupal_set_message($this->t('Created the %label Customer.', [
          '%label' => $entity->label(),
        ]));
        break;

      default:
        drupal_set_message($this->t('Saved the %label Customer.', [
          '%label' => $entity->label(),
        ]));
    }
    $form_state->setRedirect('entity.customer.canonical', ['customer' => $entity->id()]);
  }

  public function customer_note_form_js_submit($form, $form_state) {
    $message = Message::create(['template' => 'customer_note', 'uid' => \Drupal::currentUser()->id()]);
    $customer_id = 1;
    $message->set('field_customer_ref', $form_state->getValue('customer'));
    $message->set('field_note', $form_state->getValue('note'));

    $message->save();

    return $form['note_form'];
  }

}