Hi @Yihui Sun-MSFT ,
Thanks for getting back. Yes you are correct we are using Web API (Client B) as the Service Layer. However the Call from A (Web Portal) to B is through client side ($.Ajax).
The External Client (C) is a public enitity which we are integrating with through their SDK i.e. Call from B to C is from the WebAPI through the external client's SDK. So we are not using HttpClient to send request between Client B & Client C.
So the whole idea is for the Client C to have a Callback to Client B (Web API) in order to provide with the Response once they are done.
Thanks.