Yeah, I couldn't believe it either, so I just tried and it worked fabulously and without issue. You will need to have a validated custom domain on your app service (but why would you try to do this if you didn't) and then it all just works. The only things to note are:
- No naked domains (e.g. http://example.org won't work but https://www.example.org is fine)
- No wild cards (but you can create one certificate per custom domain)
- Needs a Basic plan at a minimum to use certificate
This is truly awesome and a big plus to Azure App Service as I always felt like adding a custom domain only got me half way to where I wanted to be, because a custom domain without https was not quite good enough. Prior to this you couldn't secure the domain without paying to get a certificate or venture into the world of Let's Encypt (a great service but ultimately a little bit of work). Now you can literally have your custom domain and secure it too.
More details at https://docs.microsoft.com/en-us/azure/app-service/configure-ssl-certificate if you're interested.
