how add add menu in wordpres admin side bar

<?php
/*
Plugin Name: how add add menu in wordpres admin side bar 

*/


// now add admin menu
function submenu_part2()
{
?>
<div id="wrap">
<h1>Sub menu 2 </h1>
</div>
<?php
}
function submenu_part3()
{
?>
<div id="wrap">
<h1>Sub menu 3 </h1>
</div>
<?php
}

function submenu_part1()
{
?>
<div id="wrap">
<h1>Sub menu 1 </h1>
</div>
<?php
}
function admin_menu_page()
{
?>
<div id="wrap">
<h1>how add admin menu</h1>
<h2>add sum new submenu</h2>
</div>
<?php
}
function add_admin_menu_sidebar()
{
add_menu_page("how add admin menu ","How add admin menu","manage_options","parents_menu","admin_menu_page","dashicons-edit", 1);
add_submenu_page( 'parents_menu',"how add sub menu","submenu 1" ,"manage_options","submenu1","submenu_part1" );
add_submenu_page( 'parents_menu',"how add sub menu","submenu 2" ,"manage_options", "submenu2","submenu_part2" );
add_submenu_page( 'parents_menu',"how add sub menu","submenu 3" ,"manage_options", "submenu3","submenu_part3" );
}

add_action( "admin_menu","add_admin_menu_sidebar");