If you don't need to use the PM email address with your calendar, operating from a custom domain email address only, there's a workaround to the issue.
Issue: During initial account provisioning, the PM account email address (username@protonmail.com) is set on the calendar and the calendar doesn't have an email address edit function. So, when you later add a custom domain email address to be used as default, you can't set the new address on that initially-provisioned default calendar.
Workaround in Calendar settings:
1. Export initial calendar to ICS.
2. Create new calendar using custom domain email address. Set as default.
3. Import ICS to new calendar.
4. Delete initial calendar.
Of course, this doesn't help users that need inviter options on the same calendar.
Jeff
If you don't need to use the PM email address with your calendar, operating from a custom domain email address only, there's a workaround to the issue.
Issue: During initial account provisioning, the PM account email address (username@protonmail.com) is set on the calendar and the calendar doesn't have an email address edit function. So, when you later add a custom domain email address to be used as default, you can't set the new address on that initially-provisioned default calendar.
Workaround in Calendar settings:
1. Export initial calendar to ICS.
2. Create new calendar using custom domain email address. Set as default.
3. Import ICS to new calendar.
4. Delete initial calendar.
Of course, this doesn't help users that need inviter options on the same calendar.