Form.ByLoftCrossSections is one of two OOTB Dynamo nodes available in Revit that produces a type Form geometry (native Revit geometry). It requires the input of a ModelCurve which you can get from either a Modelcurve.ByCurve or a ModelCurve.ReferenceByCurve. You can only use this node in a family as both the ModelCurve and Form.ByLoft nodes will only work in family environment.

If you pass a false into the isSolid you will get a void rather than a solid.

 

If you need a more complex geometry in Revit, take a look at the Form.ByGeometry node from Spring Nodes.