@peter_the_oak's answer is not fully correct. You can request consent for any subdomain you wish, provided that the root domain is verified.

For instance, if you're requesting OAuth access for www.example.com. You need to make sure example.com is verified by Google and tied to you.

The quickest way to do so is to head into Google search console and verify your domain. They'll provide you with a TXT value for you to save into your DNS entry and within 5 minutes, it should work.