How to properly send a curl post request with JSON data? or Why is my curl post request returning a 4

If you’re stuck, try httpbin.org to test your curl post request. It echoes back your request details, so you can see what’s going wrong.

Example:
```bash
curl -X POST -H "Content-Type: application/json" -d '{"test": "data"}' https://httpbin.org/post
```

If that works, your API might have stricter rules. Also, `-v` is your best friend for debugging.