How to configure Calico in cluster.yml?

I am using RKE 1.1.3 to install a kubernetes cluster.

I need to configure Calico specially, for example, I want to configure the parameter IptablesBackend of FelixConfiguration resource to be NFT, but I cannot find the way to define them in cluster.xml.

A sample or a link to the docs is appreciated.

Thanks for your help very much.

Guofeng