I'm not sure there's any API access for animations yet. Animated images, when spawned instead of dragging and dropping seem to either "break" or default to "stop looping" or default to "pause". I don't think this behavior is unique to SpawnDefaultToken, it's just a limitation of Roll20. It's one of the things, whether anyone at Roll20 has realized it or not, is holding back this system. I've tried creating animated spell effects, for example, using webms and it's just not possible at the moment.