public class MyMessageProcessor : Actor
{
    [MessageHandlerDefinition(typeof (MyMessage))]
    public async Task<IActorResult> MyMessageHandler(IMessage message)
    {
        // method body
    }
}

// Just create a message you would like to send
IMessage request = Message.CreateFlowStartMessage(new MyMessage());
// Define result message you would like to receive
ICallbackPoint callbackPoint = CallbackPoint.Create<ResultMessage>();
// Now, send the message. No need to know actor address, ID or anything else!
IPromise promise = messageHub.Send(request, callbackPoint);
// Wait for result
ResultMessage result = promise.GetResponse().Result.GetPayload<ResultMessage>();