Surfacing tab→Curves group→Curves list→Keypoint Curve 
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Specify the points you want to use to define the curve and then click the Accept (checkmark) button on the command bar.
Using the options on the command bar, define conditions for the endpoints on the curve.
On the command bar, click the Preview button.
Finish the feature.
Tip:
You can construct the curve using keypoints on existing elements or points in free space.
During the Select Points step, you can use options on the command bar to construct either an open or closed curve.