Add custom post types to submenu

functions.php

<?php //remove this line

add_filter( 'wp_get_nav_menu_items', 'cpt_locations_filter', 10, 3 );

function cpt_locations_filter( $items, $menu, $args ) {
  $child_items = array(); 
  $menu_order = count($items); 
  $parent_item_id = 0;

  foreach ( $items as $item ) {
    if ( in_array('locations-menu', $item->classes) ){ //add this class to your menu item
        $parent_item_id = $item->ID;
    }
  }

  if($parent_item_id > 0){

      foreach ( get_posts( 'post_type=cpt-post-type-here&numberposts=-1' ) as $post ) { //edit cpt slugh here
        $post->menu_item_parent = $parent_item_id;
        $post->post_type = 'nav_menu_item';
        $post->object = 'custom';
        $post->type = 'custom';
        $post->menu_order = ++$menu_order;
        $post->title = $post->post_title;
        $post->url = get_permalink( $post->ID );
        array_push($child_items, $post);
      }

  }

  return array_merge( $items, $child_items );
}