Customize message field on Facebook Share

This functionality was briefly reinstated sometime after 2012, by means of using p[title]=title&p[summmary]=summary in the URL's GET parameters. Take a look: How to pass a parameter like title, summary and image in a Facebook sharer URL.

But by mid-December, 2013, years after this question or answer, the functionality was AGAIN yanked from FaceBook.

To quote the blog post by Ibrahim Faour, Facebook Team, April 24, 2014...

We have changed the behavior of the sharer plugin to be consistent with other plugins and features on our platform.

The sharer will no longer accept custom parameters and facebook will pull the information that is being displayed in the preview the same way that it would appear on facebook as a post, from the url OG meta tags.

Few months ago such possibility has been removed from the FB API. So you just cannot

All the possible parameters you can found at: http://developers.facebook.com/docs/reference/dialogs/feed/