How to integrate Poynting Vector on a face

Hi, I am wondering how to calculate the integration of Poynting vectors on a specific face. I tried to draw a vaccum box and select a face then <Edit>-><Create object from faces>; When I enter the calculator, and save real poynting into stack, I found the object I just created not appear in the geometry selection options. Therefore I cant foward with integrating the poynting vectors.

Second, what solution type and exitation should be used to calculate metalic shielding effectiveness in a plane wave illumination senario?

Is it possible to provide any clue?

Cheers,