Scrape data from web page with HtmlAgilityPack c#

Try this:

public static string Download(string search)
{
    var request = (HttpWebRequest)WebRequest.Create("https://webportal.thpa.gr/ctreport/container/track");

    var postData = string.Format("report_container%5Bcontainerno%5D={0}&report_container%5Bsearch%5D=", search);
    var data = Encoding.ASCII.GetBytes(postData);

    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";
    request.ContentLength = data.Length;

    using (var stream = request.GetRequestStream())
    {
        stream.Write(data, 0, data.Length);
    }

    using (var response = (HttpWebResponse)request.GetResponse())
    using (var stream = new StreamReader(response.GetResponseStream()))
    {
        return stream.ReadToEnd();
    }
}

Usage:

var html = Download("ARKU2215462");

UPDATE

To find the post parameters to use, press F12 in the browser to show dev tools, then select Network tab. Now, fill the search input with your ARKU2215462 and press the button.

That do a request to the server to get the response. In that request, you can inspect both request and response. There are lots of request (styles, scripts, iamges...) but you want the html pages. In this case, look this:

This is the Form data requested. If you click in "view source", you get the data encoded like "report_container%5Bcontainerno%5D=ARKU2215462&report_container%5Bsearch%5D=", as you need in your code.