4-port s-parameter analysis in ADS

Hello
I have a differential filter that drawn as shown below in ADS schematic:

first of all I need to check the reflection coefficient of this filter.
second I need to check the insertion loss of the filter.
I have placed 4 ports in the schematic. Do you see any problem in my schematic?
actually all four ports should be set on 50 Ohm and I don't know how I should set it.
in other Spice software I usually place a behavioral source to generate the output=out1-out2 but here I don't know what I should do...

Thanks

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