Add new Web Source
If any site like xyz.com is restricted with login / authentication. How can i add that site in Web Source as i need to crawl that site.
You can use authentication in your Web Source.
Depending on which Coveo Platform you are using, the exact details of how to implement this may differ.
CES7 (on premises): Configuring and Indexing a Web Pages Source
Coveo Cloud V1: Adding a Web Source
Coveo Cloud V2: Add/Edit Web Source - Panel
I highly suggest you use Coveo Search to find answers to your questions, as authentication to a web source is well documented.