IFTTT Integration and "special fields"

I have a gadget hooked up to TTN and with an IFTTT integration that works fine. Whenever the gadget moves, I get an email with a few bits of relevant data.

The brief description of the IFTTT integration says …

“Sends uplink messages to your IFTTT Maker Channel. If you want to trigger this integration based on sensor values, use the ‘trigger’ payload field (true or false) as a result of your decoder or converter payload function. Use the special fields ‘dev_id’ for device ID, ‘app_id’ for application ID and ‘hardware_serial’ for the DevEUI.”

How do you actually use the special fields? By putting, e.g. dev_id in one of the Value 1,2 or 3 payload fields?

Thanks

Pat