Updated 03/11/2018 [A transfer-in of a .uk domain into Azure is not currently supported as the IPSTAG is required by Nominet on the existing provider side. I assume it would be GODADDY when transferring into Azure. The Azure portal will be updated soon to support UI based migrations]

What?

If you have some domain names registered with say 123-reg or another provider and want to migrate/transfer the ownership into Azure , you can do this with the supported top level domains: com , net , co.uk , org , nl , in , biz , org.uk , and co.in (as documented here: Buy a custom domain name for Azure Web Apps.

Why?

Some of the reasons you might want to do this:

Take advantage of Microsoft Azure’s flat rate pricing, for all domains, that they have agreed with GoDaddy
Single console to control domains, DNS, traffic manager, web sites (app service) etc. etc.
Better automation/api functionality (in my opinion) than what some of the domain name hosting companies offer.

How?

There are a few blogs on the internet on how to achieve this with PowerShell using:

New-AzureRmResource -ResourceType Microsoft.DomainRegistration/domains  

like on Jos Liebens site.

Issue

However, like others had commented, I also received this error back after running appropriate PoSh:

New-AzureRmResource : {"Code":"BadRequest","Message":"Parameter domain is null or empty.","Target":null,"Details":[{"Message":"Parameter domain is null or empty."},{"Code":"BadRequest"},{"ErrorEntity":{"ExtendedCode":"51011","MessageTemplate":"Parameter {0} is null or empty.","Parameters":["domain"],"Code":"BadRequest","Message":"Parameter domain is nullor empty."}}],"Innererror":null}  

So seems there maybe a bug with this AzureRM cmdlet? I couldn’t see this domain property mentioned in the Microsoft.DomainRegistration/domains documentation.

Solution

The Microsoft Azure REST API.

There are probably other ways to initiate a domain name transfer into Azure using the REST API, but I found this way to be pretty simple.

Go to the Domains - Create Or Update page where you interact with the API from the Microsoft docs page.
Click on the ‘Try it’ button and login with your Azure AD credentials. ( I have global admin permissions in my tenant ).
Add the mandatory parameters:
resourceGroupName - where the App Service object will be created
domainName - the domain name you are migrating from another provider into Azure
api-version - I left this as default
For a domain transfer, I used the following body:
Note: some of the properties are mandatory/required

{  
  location: "Global",  
  properties: {  
   contactAdmin: "Jack Rudlin",  
   contactBilling: "Jack Rudlin",  
   contactRegistrant: "Jack Rudlin",  
   contactTech: "Jack Rudlin",  
   privacy: "True",  
   autoRenew: "True",  
   authCode: "q\\1u{b=wbY9bNT193iNS",  
   Consent: {  
    agreedAt: "2018-10-21T20:10:40",  
    agreedBy: "70.80.90.100",  
    agreementKeys: ["DNPA","DNTA"]  
   }  
  }
}  

domain_transfer_body

You should get a 202 response back if the post was successful

Note: Don’t forget to escape your JSON! Check the authCode. I had a backslash \ in mine so I had to escape it with an additional \

In the Azure Resource Group that you specified in the earlier parameters, the App Service should be listed with the domain name you are transferring:
A day or two later, the annual charge for the domain hosting service should be taken from your Azure funds:
Finally once the domain transfer has been successfully completed, you will get access to manage the domains DNS:
Post domain transfer you’ll probably want to migrate your DNS and then web services.

I quite liked using the REST API post method from the browser. In an enterprise environment, I can immediately see these benefits:

Browser supports authenticated proxies natively - PowerShell has issues with this
No need to download/install modules for PowerShell
No local administrator rights required
I guess the Azure cloud shell is similar, but that requires a storage account and has an additional cost association