I am using facebook like button (https://developers.facebook.com/docs/reference/plugins/like/), box count style. When user clicks on like it shows a pop over allowing user to add a comment and share on his feed. Can we add a default comment in that box, trying from much time but no success yet.
This is not supported by facebook. The commentbox is reserved for the users comment on the content being liked. You should instead use open graph meta-tags on your page to control how your content is displayed in the users newsfeed or wall. You can review the open graph tags here: http://ogp.me/