UGUI OnClick Event!

Most simple way I can see:

 public class LevelDesign : MonoBehaviour, IPointerDownHandler, IPointerUpHandler {

     bool buttonDown = false;
     public void OnPointerDown(PointerEventData ped)
     {
         buttonDown = true;
     }  
     public void OnPointerUp(PointerEventData ped)
     {
         buttonDown = false;
     }
     void Update()
     {
         if(buttonDown)
         {
               print ("Call");
         }
     }
 }

Side note: the OnClick event is actually called when you release the button so it would be Input.GetMouseButtonUp. It is even called after the OnPointerUp event.