After keeping us waiting for around a year, this is what HEY came up with for custom domains:
HEY and HEY for Domains are separate products with separate accounts that you manage individually. You can’t upgrade/downgrade one to the other, but you can have both (or many). And you can always link your accounts so you can see all your email in one place.
It's disappointing to know that we have to create yet another account instead of plugging in a domain name on our existing HEY for You account. 🤦🏻♂️