Direct Path to php file

Hello there.

We have transformed a markup to pimcore. in this markup there is a very special form calculator with .js calculations, .json file etc. method ist POST and is send via PHPmailer. The file that is called is sent.php

We know that pimcore can handle forms. but we need direct path to this file. how can we say pimcore to call a file on the server?

send.php is stored in web/calc/send.php
page files are in web/static

in the script.js file we call the send.php this way:
document.getElementById('redirect').innerHTML = '<form style="display:none;" position="absolute" method="post" action="/calc/send.php">

and this returns:
http://someurl.link/calc/send.php -> not found.

How to call this sent.php in pimcore correctly?

Thanks and best wishes