[WordPress] Alter nav menu items output

<?php
function wpse267737_add_nav_menu_items($nav_menu_items) {

    // get post ID from URL
    $post_id = $_GET['post'];

    // get current post data
    $link_url = get_permalink( $post_id );
    $link_anchor =  get_the_title( $post_id );

    // build new menu item
    $new_menu_item = '<li><a href="' . $link_url . '">' . $link_anchor . '</a></li>';

    // concatenate existing items and new
    $nav_menu_items .= $new_menu_item;

    return $nav_menu_items;
}
// change TEST-MENU-SLUG to the relevant menu slug
add_filter( 'wp_nav_menu_TEST-MENU-SLUG_items', 'wpse267737_add_nav_menu_items' );