Use command+e to create code region

karabinar_slack_code.json

{
  "description": "Remap Command+e to Command+Shift+c in slack",
  "manipulators": [
    {
      "conditions": [
        {
          "bundle_identifiers": [
            "^com\\.tinyspeck\\.slackmacgap$"
          ],
          "type": "frontmost_application_if"
        }
      ],
      "from": {
        "key_code": "e",
        "modifiers": {
          "mandatory": [
            "command"
          ]
        }
      },
      "to": [
        {
          "key_code": "c",
          "modifiers": [
            "shift",
            "command"
          ]
        }
      ],
      "type": "basic"
    }
  ]
}