The most straightforward method, to me, is to make sure that Auto Workplane by Face is turned off (really a moot point for this particular drawing as there are no plane facets, but it is included for completeness when someone else tries to adapt this to another task).
Invoke the Workplane by 3 Points command and click on 3 corners of the curved surface.
Set View / Camera / Plan / By Workplane (this menu hierarchy may not exist if you have a different user workspace). Alternatively, press Ctrl+Shift+W. This will set up your view so that you are looking at the curved surface with all its corners equally 'distant' from the screen (just call it 'head on').
Draw a Rectangle by snapping to two diagonal corners, then Fillet2D the corners to the desired amount of curvature.
Invoke the Simple Extrude command (you'll want to set one of the Isometric views for ease of use) and extend the rounded Rectangle over the curved shape.
Invoke the 3D Boolean Operations / 3D Intersect command (usually found under the Modify or Modify / 3D menu), follow the instructions and voila.
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