To create a custom curtain or screen in ARCHICAD, the morph tool is the perfect solution. Start by drafting the curtain shape in plan view with a spline and/or polylines. The shape needs to be enclosed, and include a complete boundary.
In 3d, use the pet palette to extrude the shape vertically for the height of the curtain element.
Holding cntlr+shift, select the faces extruded from the square polyline portions of the plan shape, and delete them.
You can vary the curtains top & bottom for a more natural appearance.
Set the morph to “smooth” to eliminate any triangulation or unwanted lines.