c#-4.0 - Google Cloud Printer /register interface using c#

Question

I am trying to implement Google Cloud Printer /register interface using c#. I'm getting the below error:

{  "success": false,  "message": "Proxy ID is required.",  "request": {   "time": "0",   "params": {    "firmware": [     "1"    ],    "model": [     "CAAGB"    ],    "update_url": [     "test url"    ],    "manufacturer": [     "TestManufacturer"    ],    "support_url": [     "test url"    ],    "name": [     "TestPrinter"    ],    "proxy": [     ""    ],    "gcp_version": [     "2.0"    ],    "uuid": [     "uuid123"    ],    "capabilities": [     "{\"version\": \"1.0\"}"    ],    "setup_url": [     "test url"    ]   }  },  "errorCode": 115 } 

Can you please suggest how to use GCP /register interface using c#?