simpleRequest.cs

using System.Net.Http;

// Create a new HTTP client which we will use to send the request
private static readonly HttpClient client = new HttpClient();

// Create a dictionary with the value(s) you want to send
var values = new Dictionary<string, string>
{
    { "firstNumber", 123 },
    { "secondNumber", 456 },
};

// Convert the dictionary above into the correct format (url encoded) to be sent over the network
var content = new FormUrlEncodedContent(values);

// Send the url encoded data via POST to a URL or IP address
var response = await client.PostAsync("https://testibrahim.free.beeceptor.com", content);

// Read the data that the URL or IP address responded with
var responseString = await response.Content.ReadAsStringAsync();