partitioned-cookie.php

<?php
/**
 * Set a cookie with the 'Partitioned' attribute
 *
 * @wordpress-plugin
 * Plugin Name: Partitioned Cookie demo.
 * Description:  Add a partitioned cookie.
 * Plugin URI:
 * Version:     1.0.0
 * Author:      Adam Silverstein, Google
 * License:     Apache License 2.0
 * License URI: https://www.apache.org/licenses/LICENSE-2.0
 */
function add_partitioned_cookie_by_header() {
	header( 'Set-Cookie: partitioned-cookie=1; Secure; SameSite=None; Path=/; Partitioned; ' );

  // Also test a non-partitioned cookie.
	header( 'Set-Cookie: non-partitioned-cookie=1; Path=/', false );
}
add_action( 'send_headers', 'add_partitioned_cookie_by_header' );