ntfy

https://ntfy.upgrayeddsgarage.com/TestNotifs

Simple POST/GET webserver, pretty much learning API calls the server
They have a public server on their own, and you can use that for free if you obey their spam rules, which is an insanely high number, like 25000 posts a day or sumn.

Documentation is great, theres like a million ways to make the thing go ding, and its satisfying as fuck to get something else to connect to it.

Its not private though, so some obscurity would be smart.