A CNAME record isn't the same as URL Forwarding.
We don't recommend you use URL forwarding. URL forwarding uses browser tricks to attempt to keep your registered URL in your browser window title. In addition to this breaking your URL structure for visitors of your site, it causes issues with your site being indexed on search engines properly. Additionally, Squarespace may not be able to identify where your pages are being served from, which will prevent us from setting cookies that will allow you to remain logged in to your site.