UnityWebRequest - login page returns a "not found"

I'm trying to send a POST form to a login validation page using UnityWebRequest. I tried using MultipartFormSection, but the object redirects back to the validation page. I tried using WWWForm instead, and the URL says that it has correctly redirected to the member page, but the content says "page not found". I already made a GET request on the login page and set the cookies it set as the request header for the POST.

Here is the code:

 var formData = new List<IMultipartFormSection>();
 formData.Add(new MultipartFormDataSection("txtnamapengguna", username));
 formData.Add(new MultipartFormDataSection("txtkatalaluan", password));
 formData.Add(new MultipartFormDataSection("btgcari", "Log+Masuk"));
 formData.Add(new MultipartFormDataSection("txtaction", "login"));
 WWWForm form = new WWWForm();
 form.AddField("txtnamapengguna", username);
 form.AddField("txtkatalaluan", password);
 form.AddField("btgcari", "Log Masuk");
 form.AddField("txtaction", "login");
 www = UnityWebRequest.Post("https://apdm.moe.gov.my/index.php?modul=action&muka=login", form);
 www.downloadHandler = new DownloadHandlerBuffer();
 www.SetRequestHeader("Cookie", headers["Set-Cookie"]);
 yield return www.Send();
 if (!string.IsNullOrEmpty(www.error))
 {
     MainController.UpdateOutput("ERROR: " + www.error);
     DestroyImmediate(canvas.gameObject, true);
     yield break;
 }
 else
 {
     string loginOutput = www.url + "\n" + www.downloadHandler.text;
     Debug.Log(loginOutput);
     MainController.UpdateOutput(loginOutput);
     DestroyImmediate(canvas.gameObject, true);
 }

Note that I'm trying to log into a third-party page in order to send data there, but since I don't manage their servers, I can't make catcher pages to make this easier.