There are no "objects" on canvas. You can only make "On Canvas clicked" event, and inside this event figure out which part of the image on canvas was clicked, either by comparing mouse coordinates, or by reading r/g/b/a values of the pixel under mouse pointer. For example, if your circle is red and the clicked pixel is red, then you can assume that the circle was clicked.
Another option that may work in some projects is to add invisible sprites - clones of images drawn on the canvas - and use them in events.