change forwarding address under hide my email ?

apple says this:

Change your forwarding address

If you use Hide My Email with a developer, you can change the email address that receives forwarded messages from that developer. You can select any email address you have on file in your Apple ID account:

1. On your iPhone, go to Settings, then tap [your name].
2. Tap Name, Phone Numbers, Email, then tap Forward to under Hide My Email.
3. Choose the address you want emails to be forwarded to.

When you choose the email address, this applies to email from all apps and developers using Hide My Email.

but there is no other address to choose, just my apple id

I have other addresses in the "containable at" box but no choice appear

I would really rather not get developer emails at my Apple ID but would prefer a secondary address

any ideas appreciated