metabox.php

<?php

add_filter( 'rwmb_meta_boxes', 'your_prefix_register_meta_boxes' );

function your_prefix_register_meta_boxes( $meta_boxes ) {
	$prefix = '';

	$meta_boxes[] = [
		'title'   => esc_html__( 'Untitled Field Group', 'online-generator' ),
		'id'      => 'untitled',
		'post_types' => ['post', 'page'],
		'context' => 'normal',
		'fields'  => [
			[
				'type' => 'text',
				'name' => esc_html__( 'Size', 'online-generator' ),
				'id'   => $prefix . 'text_gurpy2jjvdoaazxca',
			],

			[
				'type' => 'text',
				'name' => esc_html__( 'Kind', 'online-generator' ),
				'id'   => $prefix . 'text_gurpy2jjvdo',
			],
		],
	];

	return $meta_boxes;
}