Frustrating as heck! PTC does not disappoint its reputation of incomplete documentations - there is not a single completed documentation for TW available on official sites of product / PTC support sites.
After wasting a whole day, I finally found the service - AddPropertyDefinition by accident. It uses similar api as the service with same name on Things and Templates etc.But the datashape used is different. The services from Thing,Template use the property names as IsLogged, IsReadOnly, IsRemote. And this on ThingShape needs logged, readOnly, remote etc. Also (for reasons unknown) the similar service AddPropertyDefinitions is not supported by ThingShapes.
They might be sharing the complete and informative documentation only with licensed customers. But they need to understand that there are use cases where people want to take/put the data out of/in to ThingWorx and need not necessarily have the licensed instance with them.
Hope that this is not a 'creating a mandatory dependency' thing and that the TW team is just too busy in development to publish the documentation.