Magento 1.9 Rest API POST create new user

[megaprogramming]

I'm using custom Magento Rest API, to create a new user, my postman request data is:

URL: http://megashopping.am/api/rest/register/store/1

content-type: application/json

Key: firstname

but I am having this error

{
  "messages": {
    "error": [
      {
        "code": 400,
        "message": "Decoding error."
      }
    ]
  }
} 

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