ok, for that sketch, you need to activate 3d sketch. You can't really create a plane and do a 2d sketch, because you see, where the split is in the surface, that causes the height to be different by juuuuust a little bit (because the surface is 3d). It is important for these lines to be connected to the bottom edge of the surface, so a plane would only allow you to do 1 of the 2 lines. I think that maybe you could create a 2nd plane for the 2nd line, but that seems like a waste of time.
So when you click sketch, choose the origin plane XZ, this will set the general orientation of the sketch.
Then, on the sketch palette window that pops up, the last item at the bottom is "3D sketch". Check that box.
Then you can draw lines connected to the endpoints of the surface even if they are not on the same planar level. To reach the exact center of the hole, I have another 2d sketch done that you can see, on the same XZ plane, and projected the hole on it, and in this case I had to draw a line across the hole, and a point in the middle of that line, to get the center of the hole. Not sure why, sometimes projecting a hole results in a circle with the center point already included, but anyway, this worked. So in the 3d sketch, you start the line from one endpoint on the surface, to the center point of the hole, and repeat for the other endpoint of the surface.
Since these 2 lines are now connected to the surface, in your operation you can select the line, click it again to add elements to your selection, and pick the bottom edge of the surface and the other line (lead-out).
That should do it... I hope you can make sense of this explanation...