When using onsite widget component (not the Core Web UI), sharing works a little differently and to get optimal results please read more below.
When using onsite widget component (not our Core Web UI), sharing works a little differently due to who controls the web page and to get optimal results please read more below.
Because onsite widget is just a single component embedded into your main website page dynamically, it then does not manipulate the html meta tags set by your main web page where the widget resides on.
Since most of the social websites like facebook rely on open graph meta tags to be set on start of the page (in html head section), you will need to set those open graph tags accordingly yourself for that page on your website. Facebook crawler will read and parse these from your web page (hosted on your end).
If you are using content management system like WordPress, then this is easily done through a plugin which allows OpenGraph tags to be defined for a page/post, such as for example this one: https://wordpress.org/plugins/wp-og/
Once appropriate og meta tags such as image, etc, are set the sharing should work in most optimal way and show your page on facebook for example accordingly.
For more information on facebook open graph sharing, please refer to the link here: https://developers.facebook.com/docs/sharing/webmasters