I'm not a graphics expert, here are 2 ideas:
On the canvas, combine a triangle and a rounded rectangle. If you want the point on the left to be round as in diagram, you could use a portion of an ellipse or circle to create the rounded point.
Another approach would be to create the shape in a drawing program (Photoshop...) and use it as source image for a Rectangle on the canvas. If you wanted to add a edge effect (like a 3D look) use a BorderImage.