There is a short video at https://www.sawtoothsoftware.com/support/issues/ssiweb/help_movies/pass_in_info.htm
that shows an example of passing information in to the survey as part of the link.
In general, though, if my panel company calls it something like RID or PID, then I'm going to create a password or pass-in field that is exactly the same. I can then reference that field anywhere else in the survey (including the redirect link at the end) by placing it within scripting tags: [%RID%] or [%PID%]. The advantage of passwords is that you can check a box in the Options tab to Allow respondent-defined passwords. This will accept any pass-ed in password as long as it hasn't been used by someone else. Using unique passwords allows respondents to restart where they left off if they leave and come back using the same link.
So, if currently my survey was at www.brian.com/login.html, I had a studyname of Pizza and my panel company had an identifier of PID, I would give them this link:
www.brian.com/cgi-bin/ciwweb.pl?studyname=Pizza&PID=##### with instructions to replace #### with the panel ID number
Then they are going to give me something like "Send completes to www.panel.com/complete?PID=%%%%" with instructions to me to replace %%%% with the panel ID number. So, in my terminate/link settings, I would set my link for completed respondents to go to www.panel.com/complete?PID=[%PID%]