OnCreatePostStream is a TOnCreatePostStream property that represents the event handler triggered when a client thread needs to create a stream used in an HTTP POST request. OnCreatePostStream is used during execution of the thread client connection identified by ASender. OnCreatePostStream is a published property in TIdHTTPServer. 

The server implementation may assign a procedure to the event handler to allow responding to the event notification. The event handler can perform processing required prior to creation of a stream class instance. The event handler can create and assign a stream class instance to VPostStream when appropriate, or retun the value Nil. If VPostStream is unassigned, a TIdStringStream instance will be created prior to execution of a command handler for the HTTP POST request.

TOnCreatePostStream 

TIdContext