You cannot use JSON with stripe. (Not directly. There’s a plugin that can convert it to something that you can use with stripe)

But if you only have one item, you can encode it directly in API connector.

The main issue actually is that you didn’t set the content-type header (application/x-www-form-urlencoded). You may need to also switch to raw instead of JSON for body type, but from what I know, it should work in JSON mode too (default header will be overwritted)