Normally you would want to perform a Guassian Modulated CW input simulation, so that you could calculated the loss over the spectrum. In that case, when you open the Tools->Observation Area Analysis there will be an option to inspect the Power Spectrum for each Observation Area. This gives you the detected power of the total simulation time over the detectors area.
A tutorial on the subject can be found at:
Let me know if this answers your question.