How to make sensor "lastseenat" in ios.conf

I tried make template.sensor “states.device_tracker.{device}.last_updated”

Actually I wanna make the sensor “last change at” in .ios.conf
“lastSeenAt”: “2018-07-19T13:21:43.920855”,

Which is accurate for “states.device_tracker.{device}.last_updated” and ‘last change at’ in .ios.conf