Creating an Alias means giving a second name/ alternative name to an existing account which may or may not belong to the domain.
For example, if you create the ‘information’ alias and you associate it to the existing ‘info’ account, all the messages that are sent to email@example.com will be received by ‘firstname.lastname@example.org’.
Naturally the ‘information’ account does not exist, therefore it cannot be configured on the mail client nor is it available via the web.
To create an Alias you just need to click on "New Alias" in the Email Control Panel, enter the existing account to which you wish to associate the alias, and the name of the alias itself.
For the destination account you can indicate an email account of the domain or any other email account, even if not of the domain: the alias will work all the same.
For more information see the guide.