OnGetStreams is a TIdOnGetStreams property that represents the event handler signalled when stream(s) need to be created for the IOHandler. 

OnGetStreams is triggered in Open when an event handler has been assigned for the property. The ReceiveStream and SendStream properties are passed as arguments to the event handler. 

Assign a procedure to the event handler to allow responding to the TIdOnGetStreams event notification. 

Change the value in FreeStreams to False if the stream instances in ReceiveStream and SendStream should be freed by the application instead of in the destructor for the IOHandler.

TIdOnGetStreams 

TIdIOHandlerStream.Open 

TIdIOHandlerStream.ReceiveStream 

TIdIOHandlerStream.SendStream 

TIdIOHandlerStream.FreeStreams