Menu under sub menu

vueMenuloaded.php

<?php

add_action( 'admin_menu', 'prefix_admin_menu' );

function prefix_admin_menu() {
  global $submenu;
  
  add_menu_page( __( 'Project Manager', 'text-domain' ), __( 'Project Manager', 'text-domain' ), 'manage_options', 'awesome-pm', 'pm_dashboard_cb' );
  
  $submenu[ 'awesome-pm' ][] = array( __( 'Project', 'text-domain' ), 'manage_options', 'admin.php?page=awesome-pm#/' );
  $submenu[ 'awesome-pm' ][] = array( __( 'Message', 'text-domain' ), 'manage_options', 'admin.php?page=awesome-pm#/message' );
  $submenu[ 'awesome-pm' ][] = array( __( 'Settings', 'text-domain' ), 'manage_options', 'admin.php?page=awesome-pm#/settings' );
}

// For line number 8 callback function. where main single page div loaded
function pm_dashboard_cb() {
  ?>
    <div class="wrap">
      <div id="vue-app"><div>
    </div>
  <?php
}

===============================================================
Output will be: 

Project Manager ----> Main menu : cause line no- 8
- Project ---> Sub menu: cause line no- 10
- Message ---> Sub menu: cause line no- 11
- Settings -----> Sub menu: cause line no- 12