I’m trying to automatically add posts from a CPT to a submenu in Appearance > Menus.

I’ve tried following this code suggested by @Bauwb

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

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

  
  foreach ( $items as $item ) {
    if ( in_array('nestjes-parent-item', $item->classes) ){
        $parent_item_id = $item->ID;
    }
  }

  if($parent_item_id > 0){

      foreach ( get_posts( 'post_type=nestjes-post-type&numberposts=-1' ) as $post ) {
        $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 );
}

Auto add (multiple) Custom Post Types to menu

but I can’t get it to work.

Do I need to add the class nestjes-parent-item somewhere other than the custom CSS field on the menu item?

Thanks