In construct we use frames for that, so you have to draw the frame in that shape, or use a effect that does that. im not sure now which effect would do the border part in Construct, but im pretty sure there is one... by this time for sure somebody did it, if not is a official one.
You can also use blending modes and have 2 sprites overlapping each other one being round one being squared the one on top is the round one and then u do blendmode source-a-top i think. and should cut the excess of the bellow sprite.