CST - S-parameter with farfield source or nearfield source

Hi all, I am trying to solve a scattering problem involving a Tx, a scatterer and a Rx and it takes about a week in time-domain solver to calculate S21. So I want to break the problem down to two pieces,where I first calculate farfield/nearield source from the Tx and then use this farfield/nearield source to get S21. But the problem is, in the asymptotic solver I cannot find a way to calculate S21 using this farfield/nearield source and a Rx or in other solvers using nearield source and a Rx. Is there a way to find out S21 using farfield/nearield source as a port and a Rx as another port? Thankyou.

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