Can anyone tell me. I have a php file

widget-comments.php which will be loaded to root directory of my website, and following code line will be added into the page to activate the comment box.

<?php include("widget-comments.php"); ?>

Here is complete instructions link to install Comments Block

Here is a Demo Preview

where in which page and how can I add above script line.

The main page of my site is index.php.
I dont know php.

Please check snap of my web page back end, it can be helpful to you

Site where I want to add simple comment block: