Delegate the domain
Once the DNS zone gets created and you have the name servers, you'll need to update the parent domain with the Azure DNS name servers. Each registrar has its own DNS management tools to change the name server records for a domain.
In the registrar's DNS management page, edit the NS records and replace the NS records with the Azure DNS name servers.
When you delegate a domain to Azure DNS, you must use the name servers that Azure DNS provides. Use all four name servers, regardless of the name of your domain. Domain delegation doesn't require a name server to use the same top-level domain as your domain.
https://docs.microsoft.com/en-us/azure/dns/dns-delegate-domain-azure-dns