I used the Application GUID for my WCF service that is located within the AsseblyInfo.vb (VB.NET) or AssemblyInfo.cs (C#) file of my hosting application (Windows Service) as shown below:
If you bind a certificate using the IIS GUI (inetmgr.exe), then perform 'netsh http show sslcert'. You'll see the AppID of {4dc3e181-e14b-4a21-b022-59fc669b0914}, which is the AppID IIS uses, so it's the appid I use, too.