mac-yadm/.config/karabiner/assets/complex_modifications/mine.json

{
    "title": "My Custom Shortcuts!",
    "rules": [{
          "description": "Fn + = for PrntScrn",
          "manipulators": [{
            "type": "basic",
            "from": {
              "key_code": "equal_sign",
              "modifiers": {
                "mandatory": [
                  "fn"
                ]
              }
            },
            "to": {
              "key_code": "print_screen"
            }
          }]
        }
    ]
  }