As you know, the power is calculated through integration of the Poynting vector. So if you know the polarization you could calculate the power from the formula. If it is plane wave,it is easy to use abs(E)^2. If not, you need to get H. Since you mentioned P and S, it means it is in farfiled. You can use abs(E)^2 and integrate it, and then normalized with the source power to get the transmission. related scripts are
farfield3d - Script command
farfieldspherical
farfield3dintegrate
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