Login to cPanel. You can do that from the details in your email sent by Dianahost or from Dianahost Porta
From Service page, you will see your service list. Under the “Quick Action” you will see “Login to cPanel” button
Click “Addon Domains” in cPanel
Write your new domain you want to add under New Domain Name.
The Subdomain & Document Root will be automatically filled up.
Click Add Domain.
The Final task is to update the nameserver at your newly added domain.