Create custom submenu in wordpress admin

wp-custom-submenu-page.php

<?php
/*
Reference:
https://developer.wordpress.org/reference/functions/add_submenu_page/
http://shibashake.com/wordpress-theme/add_menu_page-add_submenu_page
https://premium.wpmudev.org/blog/creating-wordpress-admin-pages/
*/

add_action( 'admin_menu', 'themeaxe_menu' );
function themeaxe_menu() {
    add_submenu_page(
      'edit.php?post_type=cjsupport', // Parent page url
      __('Overview'), // Menmu title
      __('Overview'), // Menu title
      'manage_options', // Capability
      dirname(__FILE__).'/admin.php'    // Submenu page
    );
}