If you are attending the citizenship ceremony at the same local council where you will be resident, make inquiries with them whether you can register at the time of the ceremony. Perhaps, they may provide forms for all people at the ceremony.
You may also want to take a paper
voter registration form with you and submit it to them, if they are OK with that.
The reason for suggesting this is that even if you register online, the online form is one maintained by Whitehall and they will pass those details to the electoral registration office at your local council. If you are already at the local council for your citizenship ceremony, you might as well as short-circuit the process.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.