The 'line' in the neck is actually part of the design for some of the newer Kennedy half dollars and as Fred stated, is not a die crack. You can see it in pictures from the mint. Here is a link. https://www.usmint.gov/news/image-library/john-f-kennedy-half-dollar#2012
You must not know what a Die Crack is.. A Die Crack forms on the Die itself. That is the Die that is going to strike the Blank Planchet. So when the Planchet is struck the metal flows into the crack and gives that part of the coin a raised formation. An incused formation into the field is not a Die Crack.
Just found one I posted it just a few mins ago I'm finally getting there so excited I had to put the coin down cause I'm shaking lol
These. Are. Called. Rarities. For. A. Reason. They're not usually just out there lying around to be found, despite what some jabronie on YouTube might have told you.