How to add anchored models?

When building a robot with environments, I would like to mark those objects in the env as anchored. Using Parser(plant).AddModelsXXX, it seems there is no options to mark the added geometries as anchored. I also use WeldFrames to fix those geometries. But that does not automatically mark geometries attached to the welded frame as anchored.

How can I mark geometries as anchored?

Thanks.