Switch back to the browser window which contains the Azure portal.
Click the + icon next to Add hostname.
Type the fully qualified domain name for which you configured the CNAME record earlier (e.g. www.contoso.com), then click Validate.
Select CNAME (www.example.com or any subdomain) option under the Hostname record type header.
Click Add hostname to add the DNS name to your app.
It might take some time for the new hostname to be reflected in your app's Custom domains page. Try refreshing the browser to update the data.