Create Post Field in Meta Box Plugin

meta-box.php

<?php
add_filter( 'rwmb_meta_boxes', 'your_prefix_register_meta_boxes' );
function your_prefix_register_meta_boxes( $meta_boxes ) {
	$meta_boxes[] = array (
		'title' => 'Related Post',
		'id' => 'related-post',
		'post_types' => array(
			0 => 'post',
		),
		'context' => 'normal',
		'priority' => 'high',
		'fields' => array(
			array (
				'id' => 'related_posts',
				'type' => 'post',
				'name' => 'Related Posts',
				'desc' => 'Choose 3 Related Posts',
				'post_type' => array(
					0 => 'post',
					1 => 'page',
				),
				'field_type' => 'select_advanced',
				'multiple' => true,
				'required' => 1,
			),
		),
	);
	return $meta_boxes;
}