bat-8.x-1.x-dev/modules/bat_booking/bat_booking_example/bat_booking_example.module

<?php

/**
 * @file
 * Some description.
 */

/**
 * Implements hook_theme().
 */
function bat_booking_example_theme() {
  return [
    'booking_confirmation_page' => [
      'template' => 'booking_confirmation_page',
      'variables' => [
        'header' => NULL,
        'form' => NULL,
      ],
    ],
  ];
}