Add domain with ‘www’ as you would any root domain. Once deployed, update the nameservers at your registrar to the address reported on the blog’s info page.
Once the blog is up, DNS set correctly, you can log into the WordPress dashboard and start working on your blog.
If the blog is already deployed (added to your account), you can do it under WordPress Settings, General menu by changing both URLs. You’ll need to close your browser and clear cache for the dashboard login to work.