Related posts with Meta Box

related-posts.php

<?php
//include WP_PLUGIN_DIR . '/meta-box/demo/demo.php';
add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
	$meta_boxes[] = [
		'title'  => 'Related Posts',
		'fields' => [
			[
				'name'      => 'Select Post',
				'type'      => 'post',
				'post_type' => 'post',
				'clone'     => true,
				'id'        => 'posts',
			],
		],
	];

	return $meta_boxes;
} );

add_filter( 'the_content', function ( $content ) {
	$related_posts = rwmb_meta( 'posts' );
	if ( empty( $related_posts ) || ! is_array( $related_posts ) ) {
		return $content;
	}

	// Use output buffering to use the template tags easier.
	ob_start();
	global $post;
	foreach ( $related_posts as $related_post ) {
		// Setup the post object so we can use template tags.
		$post = get_post( $related_post );
		setup_postdata( $post );

		// Now you can use template tags.
		?>
		<a class="related-post" href="<?php the_permalink(); ?>">
			<?php the_post_thumbnail( 'thumbnail' ); ?>
			<h2><?php the_title(); ?></h2>
		</a>
		<?php
	}
	$content .= ob_get_clean();

	// Reset the post object.
	wp_reset_postdata();
	return $content;
} );