How can I light a LED to indicate USB DFU data activity?

[MSchl.2105]

Using STM32CubeMX to make a DFU bootloader, I want to light a LED whenever USB packets are exchanged, to indicate that a DFU binary is being transferred.

...but I can't find the right callback to place my HAL_GPIO_WritePin (which makes the LED light up.) What is the callback to use?

[MSchl.2105]

I'm thinking that maybe the NOE pin (labeled USB_NOE and activated in Connectivity/USB-Device) may play a role to indicate USB data activity.

The problem is that this NOE functionality is very poorly documented, there are no code samples anywhere.