Adding to Dr Who Fan’s reply, you do not need to register your device before you start work.

The idea is that you start work and the server realises that it has not seen this device before and registers it. This sometimes works at the moment but sometimes needs manual intervention.

The (unique) ID number that is being assigned to your host when you install BOINC. It shows up within the first 50 lines or so of the BOINC client log when you start it...

Ralf

SIMPLE WAY TO GET THE "DEVICE ID" aka "COMPUTER NUMBER"
Open BOINC MANAGER
Goto PROJECTS TAB
Scroll down to the line for World Community Grid project
Click/Highlight the above project name
Click button on left side named Properties
Look for the line Computer ID To the right will be the number you are looking for, HIGHLIGHT & COPY this number with mouse
POST A NEW MESSAGE in the linked topic I mentioned several posts prior.

Yes, I already was in such situation. Shortly: I had a notebook where I wanted to refresh the operating system. I backed up BOINC data directory (of course after switching off BOINC agent) and attempted to backup of some more data, which required a couple of system restarts. It's worth noticing it was not the first time I did the operation like restoring BOINC on the same machine or move it to another machine etc, so trust me, it would work if done correctly.
Unfortunately, this time I didn't notice that BOINC agent was starting each time after PC restart, connecting to WCG servers, asking for data and of course increasing rpc_seqno parameter.

After the refresh, I restored BOINC data folder from the backup done before the operation, installed BOINC and started it. To my surprise, all BOINC tasks become detached, the machine received new ID and was registered as a new device.
After short investigation I realised what happened. Then I moved "new" BOINC data folder to another location, restored old BOINC folder from the backup again, increased rpc_seqno parameter by, like 10 or 20 (there were just a few connection attempts in the meantime, so no need to increase it more) and started BOINC again. This time the machine was recognised correctly, no new BOINC ID was created, machine received new set of tasks and started crunching - and was doing it happily for next couple of years until it's retirement.
To not waste tasks assigned to newly created BOINC ID I quickly created a virtual machine, put a portable BOINC version there, copied "new" BOINC data folder there and finished crunching these tasks. Since then, the folder is sitting in my backup, waiting for a free machine to be used for BOINC