JSON post values in particular format and response for login in android

I need to post the data through URl in this format

{
    "Authentication": {
      "Username": "test@123",
      "Password": "test@123"
    },
    "RequestType": 7
  }

And getting the response value for a particular post value in this format

{
StatusCode: 400
StatusMessage: "Order Request Type is Invalid"
}

In this how can i validate if status message is equal to Ok then only page redirect to Main Activity otherwise show the alert box. please help me if you have any idea.