We do not recommend or support connecting alias accounts.
Since aliases use the same sending account behind the scenes, it gets exposed to your provider (such as GSuite), and they can flag your sender's reputation (which would negatively impact your deliverability).
You can only add it as a separate account like you would with a new email.
E.g. if you try adding a Google alias, you will see this message:
The email accounts that you're connecting to Instantly need to be real users with paid user licenses and real inboxes.