400 errors are usually about the request body or headers. When I was dealing with curl sending JSON, I found that some APIs expect a trailing slash in the URL. Try this:
```bash
curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' https://example.com/api/
```
Also, check the API docs to see if they expect any specific fields in the JSON. Sometimes missing fields can cause 400 errors.
```bash
curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' https://example.com/api/
```
Also, check the API docs to see if they expect any specific fields in the JSON. Sometimes missing fields can cause 400 errors.