Add an anchor element to the last page of the website.

ahmedA Thank you for your comment. It actually helped me a lot. I edited the original post requesting help with a different problem. Any chance you have comments on that? You really seem to know your stuff

Thank you for your kind words. I just wish that my advisor stumbles upon your comment, as those are words I've never heard from him. :-(
The simplest way to do what you are aiming for is to use branches. Randomly assign them a group and branch to different conditions based on the group.
In one ask them questions, in the other, give them a link to the website or put the website in an iframe, and ask them to go there an comeback when they are done. Multiple instructions for both are available online.
While technically you can redirect them automatically, but for that you'll either need to end the survey, or if you choose a JS based solution, the the results will vary based on your respondent's cookie settings, both of which may mess up your data collection.
My advise would be that if you want to keep it as one survey, then embedd the website in an iframe or embedded the survey on the last page of your website.

Also, always ask a new question when your requirements change. While you may no longer need it, someone else might have the same problem in the future.