Obtaining extension identifier from Chrome Extension Web Store:

Lets consider 'Postman Interceptor' as the extension to be added to the extension repository:

Go to Google Chrome Web Store.
Search for the extension using the search bar.
From the URL address bar, select the extension identifier which is located between "/" and "?" as displayed in the image below.

How to obtain extension identifier of Chrome extensions.

Copy the identifier and paste it in the Extension Identifier field present in the Extension Repository.