In this case, you can use transmission: transmission - Script command – Ansys Optics
Usually it is not necessary to measure the absolute power, since most devices work linearly to the input power. you can use "sourcepower" to multiply the transmission to get the transmitted power:
and most likely, you do not need to get the transmission either, since the sourcepower will change when you change the simulation cross section, except that this is a periodic structure.
For single, isolated aperture, usually people are interested the farfield pattern, and normalize it to max intensity of 1.
Please let me know you want something else.