To do this, follow these steps:

1. Log into Plesk
2. Select the domain under which the mail account is configured
3. Click the Mail tab
4. Click on the "Create Email Address" button
5. Fill out the email address and password you would like and change any other settings relevant to this account.
6. Click on "OK" and this will create the email address.

After you have done this your email account will have been created.