Show CPT under CPT in admin

functions.php

add_action( 'init', function () {
	register_post_type( 'cpttop', array(
		'labels'  => array(
			'name'          => 'Top CPT',
			'singular_name' => 'Top CPT',
		),
		'show_ui' => true
	) );
} );
add_action( 'init', function () {
	register_post_type( 'cptsub', array(
		'labels'       => array(
			'name'          => 'Sub CPT',
			'singular_name' => 'Sub CPT',
		),
		'show_ui'      => true,
		'show_in_menu' => 'edit.php?post_type=cpttop'
	) );
} );