Hi,
You'll need to create an in-place mass or do it in a massing environment.
You can create a section perpendicular to the vertical element and draw a ref plane perpendicular to the wavy surface, set it as a workplane and draw a spline and extrude. But it would be more accurate to do it as follows.
1. Create two ref planes
2. First one that follows the wall, beam or whatever vertical element it is.
3. Offset it to the edge of the wavy surface.
4. Set the workplane to the the first ref plane you've created, and draw a ref spline where the wavy surface intersects the vertical element.
5. Switch the workplane to the second ref plane and create a ref spline that follows the wavy surface.
6. Create a form.
You may need to increase the number of ref. splines that build the surface to achieve better accuracy.
