将参数传递给页面模板WordPress

Question

I have a page tamplate "products"

<?php

/*
 * Template Name: products
 * Description: A Page Template.
 */

?>

I want to pass an argument from Single.php to the page template (products), So for that I create a funtion to filter query_vars :

function add_custom_query_var( $vars ){
  $vars[] = "prod";
  return $vars;
}
add_filter( 'query_vars', 'add_custom_query_var' );

then I write this url in my Single page (single.php)

<a href="<?php echo add_query_arg( 'prod', "my_arg", site_url( '/products/' ) )?>"> test link </a>

but I get Not Found URL.

Any advice ?? Thanks.