How to provide a return URL?

[CM2013]

The third-party website here is Servebase, a company that provides payment solutions.
They return data to our URL hosted at say, test.net\mypage.aspx

The form field simply has the ekey I want, and I read it - I need a way to access this from my C# app - a stand alone app.
Hence, I write it to a file with a unique id - see attached.

Once this is done, the control is back to my C# app - I need to retrive this file and read the EKEY value in it. I dont know the filename as its dynamically generated for ease of use. If I give the same filename, it never gets updated for every request.