add_filter( 'rwmb_meta_boxes', function( $meta_boxes ) {
    $meta_boxes[] = [
        'title'           => 'Testimonial',
        'id'              => 'testimonial',
        'type'            => 'block',
        'render_template' => __DIR__ . '/testimonial-template.php',
        'render_callback' => function( $attributes, $is_preview = false, $post_id = null ) {
            if ( $is_preview ) {
                echo 'This is the block preview on the back end.';
            } else {
                echo 'This is the block on the front end.';
            }
        },
        'fields'          => [
            [
                'type' => 'text',
                'id'   => 'name',
                'name' => 'Name',
            ],
            [
                'type' => 'textarea',
                'id'   => 'content',
                'name' => 'Content',
            ],
            [
                'type' => 'single_image',
                'id'   => 'image',
                'name' => 'Image',
            ],
        ],
    ];
    return $meta_boxes;
} );