They certainly don't want to make it easy, do they? The cynic in me is pretty certain that it's on an awkward, out of the way separate website for a reason. This is the link you'll need, but the page is currently down and giving a 502 gateway error. Try it again tomorrow, and hopefully the page will be up and running again. I'm currently waiting on my S21 Ultra order (so excited!) and have visited the page a few times in preparation. It's definitely correct, but is temporarily down I think.
I've just got my S21 and been trying the website all day - agreed its down.
I believe as long as you purchased the phone in September you have 60 days to make the claim... so let's hope the claim website is back up in that time. Worth getting a freebie that goes for 130 pounds on amazon..