This is a somewhat complex topic. Many browsers use a technology called CORS which makes using n8n to process forms a bit tricky.
So personally, I think the easiest way of doing this would be to use a form hosting service such as Jotform or Tally for the job. For Jotform, n8n has a built-in trigger node which is a bit easier to use. With Tally you’d have to use webhooks, so it’s slightly more tricky. Still works once you set the POST HTTP method used by them, just tested it:
There’s also TypeForm, but it’s really limited on the free plan.
Hope this helps getting started
Keep in mind that your desktop app would have to be running to receive webhooks. Also, the tunnel URL used by the desktop app is only suitable for testing purposes (it has no uptime guarantees and can change).
So if you have a long time project in mind you might want to consider using n8n cloud or self-host n8n on a suitable web server instead of using the desktop app.