Everything that I read about AdSense says that I need a website (blog, domain, etc.) which I do plan to get. I'm just not sure about the name of the domain that I want.
However, I want to be moving on, because the address verification letter from Google may take several weeks.
So, I would like to put in the domain that I think that I'm going to use. Can I do that, even though I don't own it (yet)?
Do they check if it's registered? Or are they just saying that it won't work because I can't access the HTML (yet)?
Do not register yet if your website is not ready.
Google will manually check your site whether it is eligible for Google AdSense. If they don't see any or poor content, they will decline your site and you will have to wait several weeks, maybe even months to re-apply for the same site. What you should do is finalizing your website until it's 100% ready, put content on it, and – most importantly – drive some visitors to it. At the very beginning you won't earn much anyway when you only have a little amount of visitors. When your website gets more popular, you should apply for your site and in most times your website gets approved in a few days.
You then have to wait for the letter, that's right – but you can already create ad tags, show them on your site and earn money like any other AdSense user. The letter is sent for verification and you can't request a payout before you didn't verified your address and bank account, but you will have to wait for $100 anyway (or the equivalent in your currency) since that is the minimum payout. After you achieved $100 at the end of a month, you will have to wait an additional 21-23 days to see your payment in your bank account.
So finish your website, put content on it, register for AdSense. You can wait for the letter while you already see earnings in your account.