Much quicker than what I was trying to work out. I did find what might be a bug when trying to create this, though.
1) Draw a right angled line with no curves
2) In the "Lines and shapes" panel, change the corner radius to 100
-incidentally, you can't use the arrows to change it to this... the arrows only allow you to change up to 50. As a result, if you have a corner radius of 100 and use the up arrow expecting to change it to 101, it resets to 50-
3) In the "Lines and shapes" panel, change the line from Straight to Bézier
Visually, there's no change, BUT, when to begin to drag one of the endpoints, the corner changes to square. Also, if you change to Bézier by way of Curve (Straight -> Curve -> Bézier), Bézier adopts the Curve's curve. Not sure what the "correct" result should be here, but having a curve lose it's curve when a point is adjusted seems off.
Maybe the curve should disappear immediately when selecting Bézier?