How to write a private message to a member?

How can I write a private message to person here on LINGQ or how to write on a member’s wall? I go on a person’s wall and see ‘share something’.Then I’m writing here a message but I can’t post it.Could someone help me?

Look for the left button down there next to “Post” where you can make it “all”, “friends” or “private”. After that you can press the “Post button”.

If, as I have often done it, you forget to make it ‘private’, you can quickly delete your own post (at least, that’s what one used to be able to do, don’t know whether it still works)

Thank you)