Welcome to the Community!
This is a bit of a problem. We can't tell you the site url, we don't know what you've set up.
If you don't know it, then why do you need support on it? It's not like you're actually using it!
So, two things
1) The url that Atlassian are asking you for is just the base of your site, it's not a complex part of it or a convoluted path. At the moment, almost all Cloud sites run on <something>.atlassian.net so all you need to know is the <something>
2) If you are the person who set up the site, then you have admin access to it, and if you log into admin.atlassian.net or my.atlassian.com, you'll be able to get a list of all the sites you own, which will give you the missing <something>
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