Are you attempting to use this with the clone activity, or WIM provisioning?
With VMware based clone, it isn't applicable, you will utilize the Guest Customization Spec but it will join to the default Computers container of the domain specified in the specification.
For WIM provisioning, I've had mixed results with that. If there is already a value in the sysprep of the original WIM image, it tends not to work. I've had co-workers that have gotten it to work (so they say), when the sysprep file doesn't contain that information and it is populated from the custom properties when the agent runs during the WIM deployment.