Hi billis, in that case you will want to do these steps:

Start with the circle that was used for the finger hole - you can draw a new one if you need to in the front view by picking the center of the circle on the origin and then the radius point snapping on to your "test curve" there:



Select the circle and run Contruct / Extrude, and click on the "Both sides" checkbox option so that it is checked. That will create this:



Now select the cylinder, run Edit/Trim and select your "test curve" as the cutting object:



Pick the outside part as the area to discard in Trim and that will leave you with the piece you want:



When you have an irregular outline, it is generally better to try and create surfaces in this way where you create a more simple surface first and then trim it by the outline, rather than trying to create a surface only using the outline curve all by itself - when the outline has a lot of curvy parts to it, it can be difficult to construct a surface using it directly without the surface having wiggles and folds in it as well.

Hope this helps!
- Michael