Sending a request with POST

I am trying to implement a payment provider (AdYen) in my application. When the payment is completed, AdYen will send me a notification on the HTTP POST request to the URL that I provided in backoffice. For instance:

http://myhost.com/Payment/FinishPayment/

FinishPayment is an action method in a payment controller. In this method, the call ends, but AdYen gives me the following error:

Required string "[accepted]" not in all results

Somehow I need to return the string "[accepted]" in the "notification". But how?

I saw a PHP example that just this:

function FinishPayment( ) // function is called by AdYen
{
    // Do stuff...

    return array("notificationResponse" => "[accepted]");
}

Does anyone know how I can send a response back to C # .NET MVC3 ??