Alright, I tested a bit further with a plain form, instead of the PHP cURL. Just to see how that works.
This is de PHP code to test:
$protocol = ((!empty($_SERVER['HTTPS']) && $_SERVER['HTTPS'] != 'off') || $_SERVER['SERVER_PORT'] == 443) ? "https://" : "http://";
$path = $protocol.$_SERVER['HTTP_HOST'].dirname($_SERVER['PHP_SELF']);
$Email = '
test@test.com';
$FirstName = 'Test';
$PostToUrl = $path . '/list-subscribe';
/* List ID */
$List = 'kf469p9yhp0f8';
/* Just checking the var's */
echo 'Path: ' . $path . '<br>';
echo 'Email: ' . $Email . '<br>';
echo 'FirstName: ' . $FirstName . '<br>';
echo 'PostToUrl: ' . $PostToUrl . '<br>';
echo 'List: ' . $List . '<br>';
?>
<form action="<?=$PostToUrl?>" method="post" accept-charset="utf-8" target="_blank">
<input type="text" name="list" value="<?=$List?>" />
<input type="text" name="EMAIL" value="<?=$Email?>" required />
<input type="text" name="DO_ORDERED" value="YES" />
<input type="text" name="FNAME" value="<?=$FirstName?>"/>
<button type="submit">Update</button>
</form>
The form is filled with all variables. (see screenshot)
However, when submitting the form, I get "Missing list id".