By default, when you create a Statusy status page you will be able to access it via the subdomain you choose by visiting <subdomain>.statusy.co. If you prefer to use your own down (perhaps something like status.yourdomain.com) you can optionally enable CNAME support by visiting Settings > Options.
Enter the domain you wish to use under "Custom Domain" and then click update. After you do this, you will need to create a CNAME record with your DNS provider for the subdomain you chose, for example, "status", it should point to your Statusy URL (<subdomain>.statusy.co).
If you want to disable custom domain usage you can do so using the arrow on the right of that field.