How to create a native file for Protein-Protein docking

Hi,

We are trying to dock two proteins, let say protein A and protein B. From tutorials (https://www.rosettacommons.org/demos/latest/tutorials/Protein-Protein-Docking/Protein-Protein-Docking), they say we need native but we having a hard time to understand how to create the native file. If someone can help us it would be great.