To add multi web site or store views in magento is not all that different to m1. 

There are essentially 3 sections to edit : Plesk | Magento | .htaccess

Ideally add your new domain name to your Plesk panel as a domain alias with only Web service selected. (ie disable Synchronize DNS zone with the primary domain / Mail service / Redirect with the HTTP 301 code)  This will ensure that all the settings for your main domain are automatically inherited as are settings for varnish etc.  However, this will mean you need a multi-domain ssl.  If you have separate SSL for each domain then you will need to add the new Domain as a separate domain and copy all the settings fro your 1st domain ie php / nginx and apache.

1) Decide if you want a new website / store / store view. If you are creating a new Store create a root category for the Store definition.

From the guide: https://docs.magento.com/m2/ce/user_guide/stores/websites-stores-views.html

Websites

Magento installations begin with a single website which by default, is called “Main Website.” You can also set up multiple websites for a single installation, each with its own IP address and domain.

Stores

A single website can have multiple stores, each with its own main menu. The stores share the same product catalog, but can have a different selection of products and design. All stores under the same website share the same Admin and checkout.

Store Views

Each store that is available to customers is presented according to a specific “view”. Initially, a store has a single default view. Additional store views can be added to support different languages, or for other purposes. Customers can use the language chooser in the header to change the store view.

2) In stores > All stores add the required website / store / store view as needed.

3) Note the code you used as this will tell magento what to present to the user.

Assuming that the domain 

edit .htaccess  and add the following at the end. Replacing your domain and your website code.

or for a store