CreatePost stores a new post and returns its id. There is no title to set: the title and the image shown for the post are derived from its content. A user or member caller authors the post as themselves and cannot set another author; a service caller may set the author to any user or member, or leave it unset for a system post, and may mark the post as a translation of another through its source. Making the post visible is not part of this call, see PublishPost.