I had the same problem, but think i found the place to make the request. Since the Hatch Act is handled via OSC (Office of Special Counsel) and McGahn was Special Counsel, then that is where the letter will exist.
https://osc.gov/pages/foia-resources.aspx
I think Q was giving us a hint by linking us to
https://osc.gov/resources/ha%20pamphlet%20sept%202014.pdf