A new email address with ‘Access to the Customer Panel’ option checked is created in Domains > example.com > Email Addresses > Create Email Address.
A new user is automatically created along with the mailbox in Subscriptions > example.com > Users, his role is Application User.
Is it possible to change this default user role?
No, it is not possible to change, because this role is hard-coded.
Vote on the Plesk UserVoice portal for the feature to allow changing the role.
The top-ranked suggestions are likely to be included in the next versions of Plesk.
To grant additional permissions to the Application user role after mailbox creation: