So the 2D profile curves would be like this.

One 2D profile in the Top view like this:



The other 2D profile in the Front view like this:



Here both shown in the 3D view:



Extrude one of the 2D profile curves out to make a base sheet:



Trim using the other 2d profile:



This generates the best quality surface where the underlying surface is very regular and has no pinching.



So the nonplanar 3D curve is generated by a trim or boolean instead of trying to start with it.

It can usually save some steps and be more convenient to work with solids - extrude the Top profile to make a solid cyllinder and then cut the cylinder with the side profile as the cutting object. That will also generate a trimmed extrusion for this surface but it will already be all connected up to surrounding surfaces instead of just being a single isolated surface.

- Michael