As you mentioned the documentation for ConverterRetrieveServiceContent says the '_this' argument needs to be of type ConverterServiceInstance. This is the root object for accessing the VMware Converter Server API and it is not a property of anything.
In the code provided by Plamen this is exactly what happens:
morServiceInstance.type = "ConverterServiceInstance";
morServiceInstance.Value = "ServiceInstance";
ConverterServiceInstance comes from the documentation. "ServiceInstance" is a well-known identifier for all VMware service API entry-point root objects. Once you get the ConverterServiceContent instance (that is the "content" of the ConverterServiceInstance object) you can examine its properties and navigate to additional service objects (e. g. sessionManager) and invoke their methods as well. In this manner you would not need any other well-known IDs besides the "ServiceInstance".