In order to create this curve you must have a sketch first. On the picture you can see a 3D sketch.
Activate the curve trough reference points. You will find it under the insert - curves - curves trough reference points.
On your left side a property manager opens up asking you to select the points.
Click OK and you have created a curve trough reference points. This curve can be used for modeling electrical cables and tubing.
If you decide to close this curve you can right click on the curve in the design manager tree and click edit. Than the same window appears when you were asked to select the points and there you will check close curve.
Than you get a closed curve trough reference points.