Reply To: Refund

My custom fields code:

add_filter( 'rwmb_meta_boxes', 'your_prefix_register_meta_boxes' );
function your_prefix_register_meta_boxes( $meta_boxes ) {
    $meta_boxes[] = array (
        'title' => 'books',
        'id' => 'books',
        'post_types' => array(
            0 => 'post',
            1 => 'books',
        ),
        'context' => 'side',
        'priority' => 'high',
        'fields' => array(
            array (
                'id' => 'taxonomy_2',
                'type' => 'taxonomy',
                'name' => 'Taxonomy Field',
                'taxonomy' => 'books',
                'field_type' => 'select_tree',
            ),
        ),
    );
    return $meta_boxes;
}