Activating the RFID Scanner using the Trigger

I have a system written for a clients MC9090 with RFID, using VB.NET 2008. The application is working fine, and the only thing left to do is enable the hardware trigger. At present the user has to press a button to active the RFID Scan function I've written. How do I capture the hardware trigger to call my RFID Scan function.

 

Many thanks in advance.

 

Colin