How do I add a custom field to a POST?

[tmighty2]

I am uploading a file to my server using VB6 and Winsock.

POST /upload.php? HTTP/1.0
Host: somedomain.com
Content-Type: multipart/form-data, boundary=uzTxw5X5HVmtcWrFhatyjH2QGVcMlBj2
Content-Length: 127289
--uzTxw5X5HVmtcWrFhatyjH2QGVcMlBj2
Content-Disposition: form-data; name="file"; filename="C:/Program Files (x86)/Microsoft Visual Studio/VB98/dtctlsu.oca"; realpath="C:/Program Files (x86)/Microsoft Visual Studio/VB98/dtctlsu.oca"
Content-Type: application/octet-stream
--uzTxw5X5HVmtcWrFhatyjH2QGVcMlBj2--

The upload works fine.

On the server, I would like to know the file's path on the client pc (=> "C:/Program Files (x86)/Microsoft Visual Studio/VB98/dtctlsu.oca").
That is why I have tried to add a custom field named "realpath" (realpath="C:/Program Files (x86)/Microsoft Visual Studio/VB98/dtctlsu.oca")

I have tried to get this field in my php script, but it's empty.

<?php
    move_uploaded_file($_FILES["file"]["tmp_name"], "uploads/" . $_FILES["file"]["name"]);

    $path = "uploads/";
    $name = $_FILES["file"]["name"];
    
    $fullpath1 = $_FILES["file"]["realpath"];
    $fullpath2 = $_POST["realpath"];

    echo "fullpath: . $fullpath1";
    echo "fullpath: . $fullpath2";
?>

How could I pass a custom field with my POST?

Thank you!

ps: I don't have access to php 8.1 yet, so I can't use "full_path".