Uploading Reference field with tracking number

I have a custom store and a Magento store. When I ship an order using the Order Manager either of the store's php scripts seem to be called with a URL with a couple of parameters which causes the tracking number to be updated in our system (either the custom store or Magento). I would like the reference number to be included in this URL so I can include it in my system. Is there any way to include more parameters in the URL that triggers the update?
The URL that is sent looks like:
/MyCustomShippingModule.php?Shipping_Access_Token=X XXXXXXXXXXXX&cmd=updateshippinginfo&OrderNumber=17 &TrackingNumber=XXXXXXXXXXXXXXX&Carrier=USPS&Servi ce=USPS+Priority&ShippingCost=5.60
(Token and tracking number have been replaced with X's)

I'd like to add "&Reference=refererenceNumber" on to the end of the URL.

Thanks,
Aaron