Add a domain in plesk panel

Adding a new domain is helpful in the following scenarios:

You want to create an additional website that is unrelated to any of the websites you already own, with its own name, web content, mail accounts, and so on. Note that in this scenario, unless you already have another second-level domain name registered, you will need to register one for the new website. A second-level domain name consists of a proper name and a top level domain suffix (called TLD for short), such as .com or .net. example.com is an example of a second-level domain. You may be able to register a domain name through your provider. Alternatively, you can purchase one from a domain registrar of your choice.
You want to transfer a domain already hosted at a different provider. In this scenario you may need to contact your domain registrar to change the authoritative name servers for the domain name of the website you want to transfer to Plesk name servers. You will also need to transfer website content - you can upload it via FTP or the File Manager, as described in the Uploading Content section.
You want to set up a website that will redirect visitors to a different website. Some possible reasons for setting up such redirection are listed in the Adding Domain Aliases section. You need a separate domain name for the domain alias.

To add a new domain, go to Websites & Domains > Add Domain.

Add a domain alias in Plesk panel

Adding a new domain alias is helpful in the following scenarios:

You want to make sure that the visitors can find your website regardless of the TLD they use. For example, you can register example.net and example.org, and use them as domain aliases pointing to your website example.com.
You want to make sure that visitors who mistype your domain name can find your website. For example, you can register example.com and use it as a domain alias pointing to your website example.com.
You want to change the domain name of your website but also want visitors who use your old domain name to be able to find your website. For example, you want to change the domain name of your website from example.com to anotherexample.com. You can configure the example.com name to be a domain alias pointing to your new website anotherexample.com.

To add a new domain alias, go to Websites & Domains > Add Domain Alias.