Интеграция вашей системы с мессенджерами по API

using System;
using RestSharp;
using System.Threading;
using RestSharp.Authenticators.OAuth2;

// .....

var access_token = "yourtoken123456";

var client = new RestClient("https://api.pushsms.ru/api/v1/");
client.Authenticator = new OAuth2AuthorizationRequestHeaderAuthenticator(access_token, "Bearer");

var request_params = new
{
  text = "hello world",
  phone = "71237654321"
};

var request = new RestRequest("delivery").AddJsonBody(request_params);

try
{
  var response = await client.PostAsync(request);
  Console.WriteLine(response.Content);
}
catch(Exception e) {
  Console.WriteLine("{0} Exception caught.", e);
}