Brian, the ID is assigned when you register. From page 7 of the original thread:
[size=83]The sensor ID number is only assigned once a PurpleAir unit has been registered (through the PurpleAir site) and data has started uploading to the Thingspeak cloud storage.[/size]
You have to look for your ID in this big file, search for your location, lat, longitude or whatever. https://map.purpleair.org/json
The entry will look something like this {“ID”:2883,“ParentID”: (this is my ID 2883).