That information is what SmartThings calls the device’s “fingerprint” and the device itself reports it as a numeric code at the time of joining.
You can find the details about how to get to it in the community FAQ on Edge. (Specifically point 8 in the FAQ.) It was easier to find in the old IDE, so you can also just use that for now, but it’s still findable without the IDE.
FAQ: I have no idea what Edge is. Is that a new developer tool? (2022)