[App Service] Determining the IP address for mapping a custom domain to a web app

[lostintangent]

In the portal, when you're adding a custom domain to a web app, it displays the IP address that you need to configure DNS to point at in order for the mapping to take effect (I assume this is an internal load balancer PIP?):

I'm probably missing something, but I couldn't figure out how to retrieve this IP address via the CLI. I looked in the output of az appservice web show, az appservice web config show and az appservice plan show and none of them seem to provide it.

Since the CLI has the mechanism for managing custom DNS zones, as well as managing custom domains associated with a web app, it would be awesome if you could connect the two by retrieving the IP to map your custom A record to.