Hey i think you have to add angular force also . so that it will rotate in the direction you are applying the force.
If you are totally dependent on unity physics to do the motion then I think it will not be work as you are expecting.
i would suggest that after applying simple force after tilt apply rotation in the direction with respect to speed of the skate.
I believe when a person is standing on a skateboard and performs an “Ollie” (is that it?), he will first shift his balance point a bit backwards so that the balance is on the real axle. If you absolutely wish to use Physics to simulate your skateboard (which I would never do unless I was building strictly a physics-based game), try setting your rigidbody center of mass on the rear axle.