How do I get the DAC extension to the main document from an event on a transaction field?

I have created a custom field on the Adjustment screen which I want to use to override the default location when adding an inventory item in the details tab.

How do I reference the extension to get the custom field from the summary section from an event (INTran_InventoryID_FieldUpdated) in the details section?

I am used to using the following to get the extension:

INRegisterExt iNRegisterExt = PXCache<PX.Objects.IN.INRegister>.GetExtension<INRegisterExt>(row);

but row is referring to the detail row and not the summary section. What do I put instead of row?

I hope this isn’t confusing and thanks for any help you can provide,

Phil