If I am not mistaken, currently this is not possible:
I read a post and want to contact the author- but to let that person know what I am referring to I just press Replay and change to Message; and the title / quote is already there, waiting for me to be cited
Right now we have to completely manually compose a message: make a title for it, add the recipient and make some kind of quote [text and / or link] to the post we want to talk about in private- am I right?