You can use your additional domain and point it to the root of your primary site. If your primary domain is example.com and point example.net to it and someone will access example.net, then the browser will display example.com
Log in to your DirectAdmin Account.
In the Account Manager section, click on the Domain Pointers Icon or type Pointers in the navigation filter box, and then the option will appear. Click on it.
On the top right-hand side, click on the Create Domain Pointers button.
Under Source Domain Enter your domain pointer name.
Your domain pointer will be created, but please make sure that your domain pointer nameserver is pointed to the same as your primary nameserver else it will not work.