How to get contact fields in custom activity

Question

I am new to the salesforce and I have created a custom activity(REST). In my config.json file > arguments.execute.inArguments, I want to get my contact fields.

I have following code:

{
  "arguments": {
    "execute": {
      "inArguments": [
        {
          "fName": "{{Contact.Attribute.YOPTO.First_Name__c}}"
        }
      ],
      "url": "https://backend-service"
    }
  }
}

When I get the call in my backend-service, the fName field is always empty. I got so far using this documentation.

My data extension structure is following:

What should be the correct mustache template mapping in this case to get the fName field of the contact.