How to calculate the wave's power which pass through an edge before a PML layer?

Hello,
I am now building a 2D structure which is half-infinite, which I should add a PML layer.
Wave incident to the structure and I would like to calculate how much power it remains after passing through it.
I know it is easy to use port BC to get S-parameter to get the Transmission, Reflection and Absorption, but if there is a PML layer, I think there is no way to set a port.
So is there a way to calculate the power at an edge?

Thanks a lot!
Han