How do I get the object outline in the sketch

Hello,

I am a student, and I have a problem. I have an extruded cylinder from the top plane and another plane which goes through the cylinder at an angle. If I use section view, the outside shape of the cylinder is some curve/ellipse. How can I get that shape into a sketch on this angled plane?

Similar as on this photo, except that it is a cone. How could I get that outline of the cone on the angled plane into the sketch?