I am able to post a reply to share point discussion with help of below link.
REST Call to Post a Reply to a Discussion Board Topic w/o JSOM
but here for posting a message ContentTypeId is required.As of now am getting it from existing replies that have added directly from share point site when a discussion is created.
But when i start to post a first message through sharepoint REST API,it fails because ContentTypeId is undefined.
Is there any way to find ContentTypeId for replies?