Custom email domain: email not available in "From" dropdown box in iCloud+ or Mail app
I followed instructions (Add an email domain you already own to iCloud Mail on iCloud.com - Apple Support) and added a custom email such as xxx@mydomain.com in my iCloud+ custom email domain settings. It shown up in the Custom Domain. I can receive email from any external address. However, I cannot find the email address when I and trying to send an email using the Mail app in MacOS Ventura 13.2.1. The custom email does not show up in the From dropdown. Only my iCloud email and "Hide My Email" options show up. I have also tried the iCloud website as well, same issue.
What am I missing? Appreciate help please!
MacBook Pro (2017 – 2020)