Just use the "before page loads" trigger.
When you are done and have it working, add code a the top of the page for security:
<?php
if (WA_isValidPayPal()) {
$_POST["Submit_order"] = true;
} else {
die();
}
?>
WA_isValidPayPal() is a function that is included in eCart that will verify that the post is coming directly from paypal. You may have to remove that during sandbox testing.