To add a domain in Plesk, you can follow these steps:

1. Log in to your Plesk control panel.

2. Navigate to the "Websites & Domains" tab.

3. Click on the "Add Domain" button.

4. Enter the domain name you want to add in the "Domain name" field.

5. Choose the subscription you want to assign the domain to from the "Subscription" dropdown menu. If you have only one subscription, it will be selected by default.

6. Set the document root directory where the files for the domain will be stored. You can either use the default directory or specify a custom one.

7. If you want to enable the "www" prefix for the domain, check the box next to "www" in the "Hosting type" section.

8. Select the desired hosting settings such as PHP version, SSL/TLS support, and additional hosting features.

9. Configure the DNS settings for the domain. You can choose to manage DNS settings in Plesk or use external DNS.

10. Click on the "OK" button to add the domain.

11. Plesk will create the necessary configuration and set up the domain. Once the process is complete, you will see a confirmation message.

By following these steps, you will be able to add a domain in Plesk.