To implement a Web service method in .NET, you have to define input parameters and return type.
These procedures are described in Implementing a Web service method in Java.
By default, PowerDesigner generates the C# or VB .NET Web Service class inside the .asmx file. If you want to generate the C# or VB .NET class in a separate file and use the CodeBehind mode for the .asmx file, you have to modify a generation option: in the Options tab of the Generation dialog box, set the value of Generate Web Service code in .asmx file to False.
You can preview the .asmx file code and the WSDL code from the Preview tab of the class property sheet.