Trying to post a reply using the API

Hi there, I want to let users click a button to submit a reply to a topic they can't see.

Currently testing with an admin account and running into a problem.

Loading this URL generates the reply just fine:

<community>/restapi/vc/messages/id/589/reply?message.body=test

But using either of these post calls, I get nothing:

$.post('/messages/id/589/reply?message.body/', {value: 'test2'});

$.post('/messages/id/589/reply?message.body=test2', {});

I'm sure I've got the post code wrong, but if anyone could tell me how to fix it, that would be great.