Implement this method to return the unique Id of the service. 
   Namespace:   Autodesk.Revit.DB.ExternalService  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2013 
Syntax
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Return Value
The extensible enum value representing the Id of the service.Remarks
For all built-in external services, their Ids are all items of the enumerated type ExternalServices.BuiltInExternalServices.
Third-party services will have their respective Ids based on GUIDs. It is important to note that a service Id must be unique. It will be an error if two external services try to register with the same Id.