The notification URL to which the challenge response is sent.
This property specifies the URL to which the final challenge response is POSTed in a browser-based flow. This must be set before calling SendAuthRequest.
After the challenge interaction is complete the ACS will post data to the URL specified in this property to notify the application that the challenge is complete. The data received at this URL can be processed by calling CheckResponse. See GetChallengeRequest and CheckResponse for more details.