How do i send POST data to a website behind authentication?

Im working on a hobby project, there is this website behind authentication that allows me to generate promotional material for products (price, sale price, name etc..) i want to make a way to automate this process by taking the data from a MySQL database and passing it to the website automatically. I can read the data from the DB without a problem but im stuck on sending the POST request to the website.

The website looks like this:

Website for making the promotional material (in dutch)

I've used wireshark to find out what happens when you click the submit button ("voorbeeld tonen") and the http data is huge... i have attached the wireshark file so you can see it for yourself, basicly it inclused a lot of parameters.

This is the code i use for submitting the data:

            private void SendPost()
        {
            string url = "http://emte20.bek.nl/nl/publicationvariants/62422/refresh";
            using (var wb = new WebClient())
            {
                var data = new NameValueCollection();
                data["field[__PRODUCTnaam__#!#value]"] = "myUser";
                data["field[__PRODUCTomschrijving__#!#value]"] = "myPassword";



                var response = wb.UploadValues(url, "POST", data);
                string responseInString = Encoding.UTF8.GetString(response);
                richTextBox1.Text = responseInString;
            }
        }

when i try to execute the function it just gives me a 422 error:

> System.Net.WebException
  HResult=0x80131509
  Message=De externe server heeft een fout geretourneerd: (422) Unprocessable Entity.
  Source=System
  StackTrace:
   at System.Net.WebClient.UploadValues(Uri address, String method, NameValueCollection data)
   at System.Net.WebClient.UploadValues(String address, String method, NameValueCollection data)
   at StoreManager.dbmanagerform.SendPost() in C:\Users\joep\source\repos\StoreManager\StoreManager\DBmanager.cs:line 43
   at StoreManager.dbmanagerform.DBmanager_Load(Object sender, EventArgs e) in C:\Users\joep\source\repos\StoreManager\StoreManager\DBmanager.cs:line 29
   at System.Windows.Forms.Form.OnLoad(EventArgs e)
   at System.Windows.Forms.Form.OnCreateControl()
   at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
   at System.Windows.Forms.Control.CreateControl()
   at System.Windows.Forms.Control.WmShowWindow(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
   at System.Windows.Forms.Form.WmShowWindow(Message& m)
   at System.Windows.Forms.Form.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

If you need any more info please let me know because i am stuck. I've tried changing the url to just emte20.bek.nl but that returns a 404 Here is the wireshark file:link to file.io download