If your simulation does not involve any nonlinear effects, in general, it is not necessary to have user-defined spectrum, since the result in frequency-domain monitors is normalized to source spectrum. Please search Understanding frequency domain CW normalization
You can get the absolute result by multiplying the result with your own source spectrum. However, as far as I know, many measured results are also normalized.
Please reconsider if it is very necessary to have your own spectrum/pulse.
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