To post a reply to a topic in a stand-alone forum, send an Atom entry document containing the forum reply to the forum replies resource. Here's one easy way to do it with an asynchronous HTTP (Ajax) request using jQuery.
IBM Connections 4 documentation link: Reply to a topic
Atom API Request Details
|Content-Type : application/atom+xml|
Status Code: 201 Created
In line 7,8 must be used the same id as the one used on the URL
Example using jQuery
Note: console.log can throw errors in IE, they are there for illustrative purposes only and should be changed as required.