I would use the Bezier Shape tool and create two points. Double click on the last post to close the shape. Hit F4 to open up the shapes tab. Check the Outline Checkbox and increase the width. Go to the Add Behavior Icon and choose, Shape, Write on. This will make the shape grow from nothing to its full size. Change the length of the behavior to increase or decrease the write on speed. Hope this helps and best of luck.
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