using System;
using RestSharp;

public class Program
{
    public static void Main()
    {
        ///////////
        var client =
            new RsestClient("https://api.ultramsg.com/instance950/messages/chat");
        var request = new RestRequest(Method.POST);
        request.AddHeader("content-type", "application/x-www-form-urlencoded");
        request
            .AddParameter("undefined",
            "token=95041eyo9eqixrsi&to=14155552671&body=WhatsApp API on UltraMsg.com works good&priority=10&referenceId=",
            ParameterType.RequestBody);
        IRestResponse response = client.Execute(request);

        ///////////
        Console.WriteLine(response.Content);
    }
}