I'm building an Azure Marketplace template that will build isolated tenancy (by definition) for a web app, but create a custom domain name based on the purchasing company (e.g.: 'acme') as a sub-domain to my product's apex domain (e.g.: 'product.com'). The specific website for the customer would then be 'acme.product.com'.
The company can specify what they want the sub-domain element to be, but I would like to verify that it is not taken before starting the ARM template deployment. That is, I would like verification to happen as part of the Azure Marketplace experience, rather than come back as a 'subdomain already exists' error.
Has anyone done this, or can explain how to do it please?