CST - What is De-embedding the port in CST studio and How to perform itin simulations?

I am starting to design a diplexer with a resonator junction. Following the e-seminar tutorial for CST Studio Suite ​​​​​​​ user, tutor mentions about de-embedding the port step to achieve zero group-delay at center frequency but I don't know how to perform it.

Could you explain and show me how to do it? thanks