How do I get sessionid to make reverse-geocoding calls in .NET?

Question

Previously, I had an application that has a map, and I was able to call map.GetCredentials to get a sessionId that I could use to make a couple of location calls to get an address from lat/lon. But what if I want to do the same thing in a Windows service (C#) that does not have a map? Is there a way to get a sessionID so that I can pass it here...

string locationsRequest =
                    String.Format("{0}://dev.virtualearth.net/REST/v1/Locations/{1},{2}?o=json&inclnb=1&key={3}",
                        urlPrefix, lat, lon, bingKey);

Thanks.