Hi Ryan,
I do not recommend you purchase an App Service Domain due to many issues with it. Please purchase a domain directly from a domain registrar instead.
A domain purchased directly from a registrar works well with all of the Azure services that support custom domains. In my experience you can purchase a new domain from a registrar and start using it with Azure services immediately.
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP