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.

Click on 'Edit mail and FTP Users'

Click on 'Add a user to this server'

Type user name , and password. Then click button 'create'