What would be an example of posting a new comment

not considering authentication but what would be the proper format of a curl statement at the terminal to post a new comment for say node 13?
I have tried a number of different ways but still not getting the comment to POST.

eg
curl -H "Accept: application/json" -H "Content-Type: application/json" -X POST -d '{"type": "comment", "subject":"Test Rest 1", "comment_body":"Test Body", "nid":"13"}' http://screenshots.localhost.com/comment
Thanks
Al