1) it won't appear in the URL. You need to create that URL yourself. Especially as the panel actually needs to add this attribute to the URL, because they will do that most probably dynamically. You just need to tell them the forma..
2) Create a short text question, give it the code "ac" for example., go to the panel integration and create a new URL attribute, call it "ac" as well.
Wenn you now use your survey URL, eg.
domain.tld/index.php/surveyid/lang/en/
the panel needs to add the URL attribute at the end of this URL, for example with ?ac=6785994 or /ac/6785994 as I described above and is described in the link I posted.
When you call this URL you will see that the short text question will be filled with the code 6785994. Check also in the response table and you will see that the code is saved. Once you have tested this, hide the question in your advanced settings and make another test to check if the code is still recorded.
Then you should be set. Maybe let the panel make a test link and try to access your survey, then check again if it the code has been saved in the database.