How to retrieve subscriber key value in backend activity?

Hi! 

I have a backend application that my user imports into his 

Journey Builder

.   

Before he 

executes the messages he selects a field to be the subscriber key

.   

I'm trying to read this field (subscriber key) on my backend application (which is using 

postmonger

 to communicate with SF). This is what I tried so far: 

1. Inside my `config.json` file, I added this:  

{
  "workflowApiVersion": "1.1",
  "metaData": {
    "icon": "img/icon.png",
    "category": "message"
  },
  "type": "REST",
  "arguments": {
    "execute": {
      "inArguments": [{
        // a bunch of fields
        "subscriberKey": "{{Contact.Key}}"
      }]
    }
  },
  "configurationArguments": {...},
  "schema": {
    "arguments": {
      "execute": {
        // a bunch of fields
        {
          "subscriberKey": {
            "dataType": "Text",
            "isNullable": true,
            "direction": "In"
          }
        }
      }
    }
  }
}

My application is using JWT and is decoding it correctly. Even the oAuth is working flawless to execute and update Data Extensions. I just need that subscriberKey value. 

This is what I receive on the token (after I decode it)  

{
  "decodedToken": {
    "inArguments": [
      // a bunch of fuilds
      // No subscriberKey here
    ],
    "outArguments": [],
    "activityObjectID": "<uuid1>",
    "journeyId": "<uuid2>",
    "activityId": "<uuid1>",
    "definitionInstanceId": "<uuid3>",
    "activityInstanceId": "<uuid4>",
    "keyValue": "<uuid5>",
    "mode": 0
  }
}

Unforunately it did not work. 

Thanks for all the help!