These are public posts tagged with #httpshortcuts. You can interact with them if you have an account anywhere in the fediverse.
Amazing how easy was to write an android app for sending files to #0x0 and then retrieving the link.
The answer is: you don't do it. You just use the excellent #httpshortcuts (from fdroid) with the URL (http://0x0.st), the method (POST) and the expected form data (parameter "file", the contents of the upload).
This creates a shortcut that you can select in the share menu of any app, and then you can share back the result URL to any other app.
Furthermore, you can use #javascript for customize further some parts of the process.