I would suggest the following article. https://optics.ansys.com/hc/en-us/articles/360050995394
The target_fom is simply an option used for plotting, I believe that what you want is the target_T_fwd to specify the output power.
target_T_fwd: float or function
A function which will take the number of Wavelengths points and return values [0,1]. Usually passed as a lambda function or a single float value for single wavelength FOM. To specify a more advanced spectrum one can define a function, it may be helpful to use, numpy windows as a template.
A target value for the figure of merit. This will change the behavior of the printing and plotting only. If this is enabled, by setting a value other than 0.0, the distance of the current FOM is given. Default = 0.0