Hey Tomas,
No I believe that "should you wish" is correct, it's kind of like saying
"You have the ability to message a moderator privately, if you want"
but without the comma.. I think that that tool tip is from back when you couldn't message anyone except a moderator or system admin. of course now you can send a message to anyone you please.
But I'm not prefect at grammar so i could be wrong. ;)