register-endpoint.php

<?php
/**
 * Add a new /part/ endpoint. Access the value of part by
 * using get_query_var( 'part' ) in a theme template.
 */
add_filter( 'init', 'h1ep_add_rules' );
function h1ep_add_rules() {
    // this will register the endpoint for all WordPress URLs
    add_rewrite_endpoint( 'part', EP_ALL );
}
?>