Hi @Mariano Watson ! I know this is a bit late and maybe someone else can use this in the future, but this is easily done in two ways depending on what form of 'delegation' you mean. If you just want www.hello.com to point at a service not in Azure, you just create an Alias (CNAME) record set called 'www' and point it at the DNS value for the external service (ie: hello-host.ourservices.otherhost.tld) . Alternatively, if you want another service to actually manage and maintain the records for the WWW as a Zone, you create a Name Server (NS) record set called 'www' and put in the Name Servers of the company that is hosting it (ie: ns1.otherhost.tld ; ns2.otherhost.tld).
In either instance. hello.com. can still be an Azure DNS public zone and you can have a one.hello.com and other Azure services using it (as long as they don't try to use 'www').