Use hotspots to make only a part of the item active. A hotspot can be used to link the area to an URL, or define active area that will react to mouse events.
Solution
Draw a rectangle (or other shape) and define OnClick (or OnOver and OnOut) action for the shape.
To make the shape invisible:
Select the shape
Choose "Item" > "Fill Style" and change the style to "No Fill"
Choose "Item" > "Line Width" and change the line width to 0
To select the shape you have to click shape border.