I am looking at the post install instructions here after putting up my first bridge:

and thought I would try out the bridgeline for it.

Bridge obfs4 <IP ADDRESS>:<PORT> <FINGERPRINT> cert=<CERTIFICATE> iat-mode=0

I have all the parts mentioned in the text except for <PORT>.  I have ExtORPort set to auto, so I never picked a port in my torrc. 

How do I get there from here?

TIA,

--Torix