OnCreatePostStream is a
TOnCreatePostStream property that represents the event handler triggered when an executing client thread needs to create a stream for use in a POST request. The server implementation can assign a procedure to the event handler to allow responding to the event notification, and top perform processing required prioer to creation of the stream class instance. If the stream class in Nill or Unassigned, a stream will be created prior to executun of a command handler for the request.