How do I set an offset to a horn that is illuminating a reflectarray antenna considering the Floquet port?

The initial incident angle between the feeding horn and the reflectarray antenna is zero degree. How to change that to 15 degree or 30 degrees? In waveguide port modelling, we click the Port and manually set the polarization angle. However, in the Floquet port, how to set the polarization/offset angle manually?