Custom Wordpress endpoint permalink to do whatever you like

my-wordpress-permalink-api-endpoint.php

<?php
add_action('init', 'add_api_endpoint');

function add_api_endpoint()
{
    /**
     * Set up the endpoint URL my-api
     * argument setup
     * /my-api/v1/
     *            |_  _my_api_key / _api_request /
     *                
     */
        
    $regex = 'my-api/v1/'; // the beginning of the API request
    $regex .= '([^\/]*)'; // match[1] = _my_api_key (required)
    $regex .= '/?'; // may or not be a leading forward slash, example: if current url is http://www.domain.com/my-api/v1/UzGhdsJF3199 or http://www.domain.com/my-api/v1/UzGhdsJF3199/
    $regex .= '([^\/]*)?'; // match[2] = _api_request *(optional, but there really should be something to tell the api what to do)
    $regex .= '/?'; // may or not be a leading forward slash
    
    $location = 'index.php?_my_api_key=$matches[1]&_api_request=$matches[2]';
    $priority = 'top'; // use 'top' here or WP will send you to the page if 'bottom' is used;

    add_rewrite_rule($regex, $location, $priority);
}

add_filter('query_vars', 'safe_query_vars');
/**
 * Specify safe variables to use with the endpoint. If a variable is not specified, it will not be available to $this->add_api_endpoint()
 * @param  array $vars an array of query vars
 * @return array       an array of query vars
 */
function safe_query_vars($vars)
{
    $vars[] = '_my_api_key';
    $vars[] = '_api_request';
    return $vars;
}

add_filter('template_include', 'handle_api_request', 99);

/**
 * Handle the API requests and route where necessary
 * @return string wp template URL which should always be returned for non-api calls
 * @example my-api/v1/{YOUR_API_KEY}/{API_REQUEST}/
 */
public static function handle_api_request($template)
{

    $my_api_key = get_query_var('_my_api_key', null);
    $api_request = get_query_var('_api_request', null);
 
    die($api_request);
    return $template; // must be returned, always!
}