post_id used by item menu Page

Hi,

I use a shortcode into the title of menu item that display a Custom Field Value (this is the actual main menu into the top bar -> submenu "Services")
At the moment I have to set either the field + post_id [field "my_key" post_id=1] or only the field
[field "my_key"] and it will display the field value of the current post.

I would like to add a third possibility: In fact, The shortcode is used in a menu item type Page that contains Custom Field Value I want to display is stored in the same post_id.
I would like use a shortcode like that [field "my_key" post_id=item_menu_pageid] or directly by hook for a specific item menu.

this is the function:

add_shortcode('field', 'shortcode_field');
 
function shortcode_field($atts){
     extract(shortcode_atts(array(
                  'post_id' => NULL,
               ), $atts));
  if(!isset($atts[0])) return;
       $field = esc_attr($atts[0]);
       global $post;
       $post_id = (NULL === $post_id) ? $post->ID : $post_id;
       return get_post_meta($post_id, $field, true);
   }

from https://www.isitwp.com/get-custom-field-value-with-shortcode/

could you help?