This response from Jon Mayfield suggests it will auto-detect if Zwift thinks your machine is up to it:
https://forums.zwift.com/t/ultra-graphics-settings/9582/2
It’s an old answer, so might not be the same now. Even with a great GPU, it’s possible your CPU could be a limiting factor.
If you want to try anyway, this article describes how to use a config file to specify your settings.