Indeed Henk and Brian, the sensor ID will not be available (or assigned) until the PurpleAir unit has connected to the Internet and has started sending its data to the Thingspeak cloud storage.
Henk, if you still cannot find the correct IP address that your router has assigned to your PurpleAir unit, may I recommend that you install the free NirSoft utility Wireless Network Watcher - in the ‘Advanced Options’ (the F9 key), set the program to scan the IP addresses that your network is using. You should see something about the ‘PurpleAir-Sensor’ with the IP address it has been assigned.