How to map multiple subdomains into one app service in azure?

I have existing app service with subdomain app.example.com. I want to have another subdomain called client.example.com which is also pointing to app.example.com

Is this possible in Azure DNS?

Yes you can create the above requirement as shown below image,

• if you have already bought your domains configure DNS A and TXT records of both domains to point to your Azure web app.

NOTE:- Please ensure that the SSL certificate to be import for your custom domains includes your app service FQDN in SAN(Subject Alternative Names) section in the certificate for it to be shown as https secure.

Is this possible in Azure DNS?

• Yes its possible ,Please refer this MS DOC to Delegate an Azure DNS subdomain

For more information please refer this MS Q&A discussion|Hosting multiple domains under one app service