Azure Migrating Website Hostnames
There is documentation for this exact scenario: https://docs.microsoft.com/en-us/azure/app-service/app-service-custom-domain-name-migrate
You can create a TXT record with host:
awverify.register.tourofmargaretriver.comand value:
cyclingeventuresregistration-production-webapp.azurewebsites.netThis verifies domain ownership without directing traffic to it.