using System;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var foo = new Foo { KibanaIndex = "whatever" };

        var resolver = new CamelCasePropertyNamesContractResolver();

        var settings = new JsonSerializerSettings { ContractResolver = resolver };

        var json = JsonConvert.SerializeObject(foo, settings);

        Console.WriteLine(json);

    }

}

​

public class Foo

{

    [JsonProperty(PropertyName = "KIBANA_INDEX")]

    public string KibanaIndex { get; set; } 

}