using System;

using System.Text;

using System.Net.Http;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        string API_KEY = "test_0dc8d51e0acffcb1880e0f19c79b2f5b0cc";

        HttpClient httpClient = new HttpClient();

        var authHeaderBytes = Encoding.ASCII.GetBytes(API_KEY + ":");

        httpClient.DefaultRequestHeaders.Authorization = 

            new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", Convert.ToBase64String(authHeaderBytes));

        var requestPayload = new Dictionary<string, string>

        {

            { "to[name]", "John Doe" },

            { "to[address_line1]", "123 Main St" },

            { "to[address_city]", "San Francisco" },

            { "to[address_state]", "CA" },

            { "to[address_zip]", "94107" },

            { "front", "<h1>Hello {{name}}</h1>" },

            { "back", "<h1>Hello {{name}}</h1>" },

            { "merge_variables[name]", "Harry" }

        };

        var encodedRequestForm = new FormUrlEncodedContent(requestPayload);

        var APIResponse = httpClient.PostAsync("https://api.lob.com/v1/postcards", encodedRequestForm).GetAwaiter().GetResult();

        var addressString = APIResponse.Content.ReadAsStringAsync().GetAwaiter().GetResult();

        System.Console.WriteLine(addressString);

    }

}