To get a link preview you have to use the link icon on the bottom right hand corner.
If you don't use that, then the system just puts the link in as a clickable link but won't give you a preview. Currently you have to use i.framely or another program that will do that for you.
They are talking about making a the software able to do what you're requesting...but I think that's in a later update.
Hopefully that was the answer you were looking for...