In frequency domain solver, the field strength is determined by the power of port. You might refer to the concept of S-parameter, where you would figure out the result calculate in frequency domain are normalized that the power of port has no influence on your result.

Jihong Zhang Thank you for the answer.The material I am trying to simulate is non-linear in nature, the optical constants are strongly dependent on the field. Which is why I want to alter the excitation strength. I found out that one can feed a custom input signal from the new signal tab under the simulation tab.