gistfile1.txt

<?php


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

/* set up rewrite rule  */
add_action('init', function(){
    add_rewrite_rule('(customendpoint)$', 'index.php?customendpoint=true', 'top');
});

/* if customendpoint is set redirect */
add_action('pre_get_posts', function($query){
    $custom_page = get_query_var('customendpoint');
    if ( $custom_page ) {
        include(get_stylesheet_directory() . '/customendpoint.php');
        die;
    }
    
});