You free domain URL that you can share is displayed in the website settings (only without "www") :

Remember that it is a branded subdomain and can not be used with "www". WWW is reserved strictly for first level domains.

Otherwise you will get a warning like this:

You can customize the subdomain however to any available name.

WWW is a subdomain on its own, so using it will your free subdomain will result in two subdomains and 3 dots in a domain, which is technically impossible and will give "insecure website" warning.

If you want to use www or w first level domain, you can connect your existing domain or buy one from one of our recommended domain providers.