How do I auto fill a customfield?

[HungryH]

I have set up a custom field and would like to auto-fill it. The link to the order page is

 

order.php?step=2&pid=4

 

I would like to change it to

order.php?step=2&pid=4&new_id=whatever

The value of $new_id could then be fed into customfield[1].

 

I think I'm right in saying I need to change

templates/orderforms/default/order-stepthree.tpl but I do not know how to. I was thinking on these lines:

{$new_id = $_GET['new_id'];}
{if($new_id != "") {
   $customfield[1]['value'] = $new_id;
}
}

 

But I haven't got a clue. Not the standard php I'm used to. Can anyone help me?