How to register a custom menu in the admin panel

Please copy code given below and paste it in function.php

// Comment

function register_myCustom_plugin() 
{ 
 add_menu_page( 'My Custom Menu', 'My Custom Menu', 'manage_options',
 'index.php', '', 'dashicons-admin-site', 7 );
 } 

add_action( 'admin_menu', 'register_myCustom_plugin' );