My custom sub-domain not getting Live - can't it be automatic?

Have you already checked this article? A 403 forbidden error is a common issue when setting up a new domain name, and it usually means you didn’t upload your website correctly and to the right place.

Please note that the server will look for index.php and index.html files by default. If you get a 403 error, it means that no such file was found. If you have another file that should be loaded as the main page, this would be the way to do it, but even then I would strongly suggest to update your website so it conforms to file naming standards than reconfigure the server to fit your website.