Plesk does not provide an option for creating email accounts or email forwarding for alias domains.
However, plesk provides an option of making mailboxes alias. See a check box for Mail Service option and check it to allow mailbox alias.
Every email account created under the primary domain is created as an alias mailbox in the alias domain.
For example woza.io is the primary domain
Truehost.africa is a domain alias to woza.io