[dannythefool]

As far as I know, you can't do this with a shape, but you can do it with a connector line.

1.) Make some straight connector somewhere on the canvas
2.) Double tap somewhere on the line you just created to add one midpoint or more, then move them around
3.) With the polygon selected, tap the info button, select "Line", and pick the second of three options (the button with a curve on it)

That'll produce curves like what you attached, but again, you won't have a fillable shape or anything, just a simple curve. You *can* however connect to this line, like to all connectors.