This page POSTs JSON to api.php. Your API key is shown only once — copy it into the desktop app (Tools → Integrations).
api.php
Paste the exact address where api.php opens in the browser (no ?r=). Example: if ping works at https://site.com/workshop/api.php?r=ping, paste https://site.com/workshop/api.php. Leave blank only if this HTML file lives in the same folder as api.php.
?r=
https://site.com/workshop/api.php?r=ping
https://site.com/workshop/api.php