I do this on my order approved/complete page by calling this php/curl function:
<?php
$aff_form_data="list=19&email=" . $_SESSION['x_Email'] . "&fname=" . $_SESSION['x_First_Name'] .
"&lname=" . $_SESSION['x_Last_Name'] . "&seq=1&del=1";
$ch=curl_init();
curl_setopt($ch, CURLOPT_URL, "http://www.mywebsiteURL.com/m/signup.php");
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);
curl_setopt($ch, CURLOPT_NOPROGRESS, 1);
curl_setopt($ch, CURLOPT_VERBOSE, 1);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION,0);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $aff_form_data);
curl_setopt($ch, CURLOPT_TIMEOUT, 120);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_FORBID_REUSE, 1);
curl_setopt($ch, CURLOPT_FRESH_CONNECT, 1);
$buffer = curl_exec($ch);
curl_close($ch);
?>