Particle under button

[DJVDJV]

Hellos. I want to play particle under button.

How I should set particle position?

I have tried:
Vector3 particlePosition = UICamera.mainCamera.ViewportToScreenPoint(transform.position);

What I have done wrong. What is right way to write this line?

[ArenMook]

Look at NGUIMath.OverlayPosition.

widget.transform.OverlayPosition(transform);