How Do I create a Webservice in WPF?

I need to create encryption method that is embedded in the web methods for all messages that will pass through the API.

 

 Sample Message to Invoke Request Session method:
RetSession sess = requestSession(string sUsr, string sPwd, string signature)
Where:
signature = (sUsr + sPwd + secretKey)

Response:

0 - Invalid Reference Number
1 - Transaction Successful
2 - Invalid Login (Username/Password)

 

 

I have created like this 

App.config

 

But am getting error in the Page : 

  //Final Process Button click
private void btn_process_Click(object sender, RoutedEventArgs e)
{

BOC_Service.RetSession sess = re
sess.



}