example karabiner app runner, put into .config/karabiner/assets/complex_modifications/

apps.json

{
  "title": "Run apps",
  "rules": [
    {
      "description": "Run Safari",
      "manipulators": [
        {
          "from": {
            "key_code": "left_command",
            "modifiers": {
              "optional": [
                "any"
              ]
            }
          },
          "to": [
            {
              "key_code": "left_command"
            }
          ],
          "to_if_alone": [
            {
              "shell_command": "open -a 'Safari.app'"
            }
          ],
          "type": "basic"
        }
      ]
    }
  ]
}