@Edward Northridge Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
Based on the shared information I have understood that you want to move the app service domain from azure to another registrar. If yes, As @Takahito Iwasa suggested you try accessing the advancement portal directly by using the shared link.
Alternatively,Transferring of domain out of Azure (not a registrar) to another registrar is supported and you can do so via API and PowerShell. You will need to get an authorization code which you can take to the registrar of choice to proceed with transferring out of your domain. you can refer to this blog post for more information on how to transfer the domain out from Azure.
Feel free to reach back to me if you have any further questions on this.