Hi,
You need to map your custom domain in Azure App Service (this is separate from Azure Active Directory custom domain). Below article has instructions:
Tutorial: Map an existing custom DNS name to Azure App Service
Once you get it working you will want to set up your TLS/SSL certificate. Link is at bottom of the article.
If the above was helpful please click Accept answer. If you have a question/concern please add a comment.
Thanks.
-TP