How Do I Send a CURL POST Request with Headers and JSON Data? or What’s the Correct Syntax for a CURL

400 errors suck. Here’s my checklist:
- Is the URL correct? No typos?
- Does the endpoint *actually* accept POST? Maybe it’s GET-only.
- Try `-H "User-Agent: curl/7.68.0"` (some APIs block "curl" UA).

For debugging, I use `curl -v | less` to scroll through the output.