[Thomas]

To do that, you have to use the composite curve system. The easiest way:

1. Open your 2D floating panel. Click on the small losange to stick it open, even when you done with the tool (near the icon to rotate this floating panel)
2. Pick the polyline and create your polyline. Don't validate after your last point, I mean, don't quit the tool
3. Without quitting the polyline tool, pick the arc circle tool in the 2D panel and create your arc circle: the first point of the arc circle is connected with the previously created polyline.
4. Don't forget to use the sapce bar with the arc circle tool to use the constraints (to have 90, 180, etc. arc circles).

I hope it helps!