Then it is possible to flatten your curled non-planar shape down into a planar curve - if you have something that you want to flatten to a world direction you can use the edit grips (the frame that appears around the outside of the selected objects in the 2D views) to squish it down in one of the 2D top/front/right views until you get "flat" snap.

But if you want to flatten something to a particular direction it can be more convenient to use the Transform > Scale > Scale1D command, since it allows you to pick 2 points to define which direction you want the scaling to happen in.

So in your case here you would select the shape you want to flatten:



Then run the Transform > Scale > Scale 1D (scale in 1 direction) command which is located in the UI here:



Now when you're running a command it's always a good idea to look in the upper-right area of the window to see the prompt for what MoI is expecting for you to do next - the first prompt here says "Pick origin point".

You already have a direction line set up for the direction you want to extrude in, pick the base of that line as the origin for the scale:



That will bring up the second prompt, which says "Pick scale direction" - pick the end of the line here:



Now at the third prompt you can pick a point to define the scale amount, or enter in a value - if you slide the mouse down towards the scale origin point you can see that the shape will gradually flatten as you approach it - for this stage you want to enter in a scale factor of 0 (type in 0 and push enter) to completely scale the curve down until it is totally flat around the base of that direction line, like this:



Now that you have a closed and also planar profile you can now select it and extrude it and get end caps like this:



See the attached mark_flattened_profile.3dm file for the flattened result.

- Michael