tooltip OnMouseOver - screen or mouse position?

I have several game objects that the user can click on (lmb) to show/hide labels; when enabled, the labels then move around the screen following the go's screen position. However, I'd also like to have a tooltip appear when the mouse is over the g.o.

Based on a couple of different postings here (1,2), I changed my code a bit to the following:

     var fmrToggle: boolean = false;
     var screenPosition :  Vector3;
     
     function OnMouseOver (){
         screenPosition = Event.current.mousePosition;
     }
     
     function OnMouseDown (){
         fmrToggle = !fmrToggle;
     }
     
     function OnGUI (){
             if (fmrToggle) {
               GUI.Label(Rect(screenPosition.x, screenPosition.y, 50, 20),"Femur", GUI.tooltip);
                }
     } 
     
     function Update (){
         screenPosition = Camera.main.WorldToScreenPoint(transform.position);
         screenPosition.y = Screen.height - screenPosition.y;
     }

but am still getting no tooltip when the mouse is over the gameobject. Any ideas would be great - thank you.