CST studio setting microwave source power

Hello,

I am making a simulation of the power distribution in a liquid, irradiated in a cavity.
(microwave studio, transient resolver)
Since the falloff will be different at different ERP's, is there a way to set the power of the signal source. My source is 6ommx60mm, gaussian.

I'd be gratefull if someone could explain this to me.

Best regards,
Steve