Custom Function Domain Error

I have a function and it's default domain is https://66c4ef207ae4885d4fc7.aw.example.com/, it works, that's fine.

But I want to add a custom domain. I want https://my-function.aw.example.com/. So I put in my-function.aw.example.com to the Domain for the Create Domain screen and hit Next and I am given the toast error. "You cannot assign your functions domain or it's subdomain to a specific resource. Please use a different domain."

Is this intentional, is it something I can disable, is Appwrite generating domains for functions that don't follow it's own rules? Or am I just misunderstanding the point of custom domains? Is what I want even possible?