You can use the Feed URL "Direct URL" option, as described on the Feed Dialog page:
You can also bring up a Feed Dialog by explicitly directing the user to the /dialog/feed endpoint:
https://www.facebook.com/dialog/feed?
app_id=123050457758183&
link=https://developers.facebook.com/docs/reference/dialogs/&
picture=http://fbrell.com/f8.jpg&
name=Facebook%20Dialogs&
caption=Reference%20Documentation&
description=Using%20Dialogs%20to%20interact%20with%20users.&
redirect_uri=http://www.example.com/response`
Looks like they no longer mention "sharing" anywhere in their docs; this has been replaced with the concept of adding to your Feed.