Get unique userID in initial URL and send it back when the "quiz" is successfully completed

Hello there :)

I'm totally new to Storyline 360, so please forgive me if the question has already been answered millions times…

I have created a quiz with Storyline. People will access the quiz thanks to a business card they will receive at an event. Thanks to the business card, they will be able to scan a QR Code containing a unique url that sent them to a landing page with a unique userID attached (let's say: https//mysite.com/landing?userID=1234). When they click on a button on that landing page, they will be redirected to the quiz URL, with a the URL parameter containing the unique "userID" (like this "?userID=1234").

Then, they will start the quiz. At the end of the quiz, I want to redirect them to a "thank you" page on my website, but my website needs to get the userID back to know who has successfully completed the quiz and store its userID (url like https//mysite.com/thank-you?userID=1234). 

I've managed to handle the website part (dynamically get the userID from URL and put it as a parameter of the button, get the userID when on the thank-you page and store it in the DB).

But now I need to make the connexion between the landing page, the quizz and the thank-you page. How can I get the userID at the start of the quiz and put it in the redirect URL at the end of the quiz ?

I hope this is clear enough :p

Thank you in advance,

Cedric