使用C#发送文件和数据错误403. CURL没有错误

I'm trying to upload a file and some data with POST request from my C# program to my server, but I receive always error 403.

The post params are "id" = folder where the file will be saved "pos" = the name of the file

So, if an user upload file "abc.text" and POST data are id="Mario" pos="first" the file will be saved in /users/Mario/first.txt

I tried to change params id and pos as GET, but I always have error 403

C# response

{StatusCode: 403, ReasonPhrase: 'Forbidden', Version: 1.1, 
Content: System.Net.Http.StreamContent, 
Headers:{  Vary: Accept-Encoding  X-Varnish: 818481486  Age: 0  
X-Cache: MISS  Transfer-Encoding: chunked  Connection: keep-alive  
Date: Thu, 18 Apr 2019 14:29:10 GMT  Content-Type: text/html; 
charset=iso-8859-1}}

My Code:

<!DOCTYPE html>
<html>
<head>
  <title>Upload your files</title>
</head>
<body>
  <form enctype="multipart/form-data" action="uploader2.php" method="POST">
    <p>Upload your file</p>
    <input type="file" name="uploaded_file"></input><br />
    <input type="input" name="id"></input><br />
    <input type="input" name="pos"></input><br />
    <input type="submit" value="Upload"></input>
  </form>
</body>
</html>
<!-- language: lang-php -->
<?PHP
  if(!empty($_FILES['uploaded_file']))
  {
    $path = "users/".$_POST['id']."/";

    if(!is_dir($path))  
        {  
        if(!mkdir ($path,0777,true))
            echo 'Error creating folder!';  
        }  

    $path = $path.$_POST['pos'].".txt";

    if(move_uploaded_file($_FILES['uploaded_file']['tmp_name'], $path)) {
      echo "Ok";
    } else{
        echo "Failed!";
    }
  }
?>

This is my C# code

using (var httpClient = new HttpClient())
{   
    MultipartFormDataContent multipartContent = new MultipartFormDataContent();
    var fp = File.ReadAllBytes("file.txt");

    multipartContent.Add(new StringContent("Mario"), "id");
    multipartContent.Add(new StringContent("first"), "pos");
    multipartContent.Add(new ByteArrayContent(fp, 0, fp.Length), "uploaded_file", "file.txt");
    HttpResponseMessage response = await httpClient.PostAsync("http://host.com/uploader2.php", multipartContent);

    response.EnsureSuccessStatusCode();
    httpClient.Dispose();
    string sd = response.Content.ReadAsStringAsync().Result;
    }
}

https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS

Use this in your php script:

header("Access-Control-Allow-Origin: *"); // wildcard allows access to all domains

Access-Control-Allow-Origin is a CORS (Cross-Origin Resource Sharing) header.

When Site A tries to fetch content from Site B, Site B can send an Access-Control-Allow-Origin response header to tell the browser that the content of this page is accessible to certain origins. (An origin is a domain, plus a scheme and port number.) By default, Site B's pages are not accessible to any other origin; using the Access-Control-Allow-Origin header opens a door for cross-origin access by specific requesting origins.