Are you not able to host your service on a server? You could use some cheap cloud servers from DigitalOcean or Hetzner and it would cost you something like $5

Are you not able to host your service on a server? You could use some cheap cloud servers from DigitalOcean or Hetzner and it would cost you something like $5 to $10 (depending on the resources you need). You might spend a bit of time initially just to set up everything but technically it woulf be easy to set up. For password protection, you can probably do some basic http auth.