If you are not adequately familiar with doing rest calls, check out this blog entry. http://trisummit.net/2012/02/06/using-rest-on-jive-sbs/
This is taken from the jive documentation. Are you saying that this does not work?
Creates a new thread in the specified community.
The subject of the new thread. body
The body of the thread's root message. communityID
ID of the community in which to create the new thread. userID
ID of the user creating this thread; pass less than zero for anonymous.
Parameter XML Payload Template
<createThread> <subject>xs:string</subject> <body>xs:string</body> <communityID>xs:long</communityID> <userID>xs:long</userID> </createThread>
ForumThread The newly created thread.
Response XML Payload Template
<createThreadResponse> <return> <!-- Contents of ForumThread --> <return> </createThreadResponse>
You can get the communityID from the admin console. May be kinda funky, but I get it by going into spaces, select the space I want the id from, select edit, and look at the URL.
You can use the API to add to a private discussion as long as the user set in the header credentials of the code has permissions to create threads in the targeted space.