Add custom code to Purchase Order Entry New Line

In Vista 8.03.409C in the Purchase Order Entry screen, when you start a new PO, then select New Line, the Buy For Inventory radio button on the Lines tab is selected. I'd like to add some code to select a different radio button. Where can I add code to make that happen...I don't know where to find the event that fires when New Line is selected. The Lines tab is populated when the form loads, but selecting the New Line option causes some code to run and select the Inventory option.

Thank you.