add_filter( 'rwmb_meta_boxes', 'book_meta_boxes' );
function book_meta_boxes( $meta_boxes ) {

  $meta_boxes[] = array(
    'title'      => esc_html__( 'Embed', 'book-pt' ),
    'post_types' => 'book',
    'priority'   => 'low',

    'tabs'      => array(
        'website' => array(
            'label' => __( 'Website', 'book-pt' ),
        ),
        'facebook'  => array(
            'label' => __( 'Facebook', 'book-pt' ),
        ),
    ),

    'fields'     => array(
      array(
        'id'   => "_website",
        'type' => 'custom_html',
        'tab'  => 'website',
        'class' => 'embed-tab',
        'std'  => 'test',
      ),
      array(
        'name' => esc_html__( 'Facebook', 'book-pt' ),
        'id'   => "_facebook",
        'tab'  => 'facebook',
        'tooltip' => esc_html__( 'Tooltip content', 'book-pt' ),
        'type' => 'url',
      ),
    )
  );

  return $meta_boxes;
}