Thanks for asking question! If my understanding is correct you want to map a custom domain name to an Azure Static Web Apps application using root domains?
To elaborate root domain are your domain minus any subdomain, including www. For example, the root domain for www.example.com is example.com.
As root domain support is not available during preview, you can make it work with a little bit of DNS trickery and the help of Cloudflare.
Please refer the blog post Configure root domains in Azure Static Web Apps for details on how to configure root domain support with a Static Web App.
Hope this helps. Let us know if issue remains.