Step 1 : Login to administration console.
https://:10000
Example : https://www.multisys.hk:10000
Step 2:
Click on Servers -> Virtualmin Virtual Servers
double-Click on your domain name.
in some of our system , it may be 'Servers' -> Virtual Email
Click on 'Edit mail and FTP Users'
Click on 'Add a user to this server'
Type user name , and password. Then click button 'create'