Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I understand that you would like to use a custom domain "au.abc.com" for your BigCommerce.
If you are planning to update the NameServers, you have to modify the configurations in BigCommerce(site) and AWS(registrar) , and not in Azure
Please note that at no point, Azure will act as your registrar.
Azure can only host DNS Zones that you purchase from a Registrar.
You have to check with the service (BigCommerce) if they support hosting your domain in their service, and modify the records from there.
However, you can use CNAME records in Azure DNS and point the requests to "au.abc.com" to <yourShopName>.mybigcommerce.com
This doc may come handy : Create the CNAME record
Please note that the above will lead to the requests directly going to your <yourShopName>.mybigcommerce.com and not via the NGINX server
Kindly let us know if the above helps or you need further assistance on this issue.
Thanks,
Kapil
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