Here's what I did.

My wave port size is 1.5mm high x 1.604mm wide.

In driven terminal solution type, I right clicked the green rectangular wave port and set it as wave port. I made GND and the surrounding ground plane references : https://imgur.com/a/HgXr9K0

The terminals from the narrow conducting transmission line were automatically identified as feed1_T1 and feed2_T1

After, I right clicked this port under excitation (in project manager), went to differential pairs, added a new pair, set ref Z to 50 ohms. I clicked "matched" https://imgur.com/a/e9NzQPj

Next, I right clicked excitations (project manager) , clicked Edit sources, and made them 180 out of phase. https://imgur.com/a/XTQ9FSr

I ran port only solution and found that port Z0 at the frequency I design feed for is very high