commerce-8.x-2.8/modules/checkout/src/Plugin/Commerce/CheckoutPane/CompletionMessage.php

<?php

namespace Drupal\commerce_checkout\Plugin\Commerce\CheckoutPane;

use Drupal\Core\Form\FormStateInterface;

/**
 * Provides the completion message pane.
 *
 * @CommerceCheckoutPane(
 *   id = "completion_message",
 *   label = @Translation("Completion message"),
 *   default_step = "complete",
 * )
 */
class CompletionMessage extends CheckoutPaneBase {

  /**
   * {@inheritdoc}
   */
  public function buildPaneForm(array $pane_form, FormStateInterface $form_state, array &$complete_form) {
    $pane_form['#theme'] = 'commerce_checkout_completion_message';
    $pane_form['#order_entity'] = $this->order;

    return $pane_form;
  }

}