If you can reach the piaware status page somehow, there will be a link there that will claim the feeder if you visit it while logged in to your FlightAware account.
If you can’t get to the status page then you’ll need to construct the URL by hand:
flightaware.com/adsb/piaware/claim/FEEDERID
where FEEDERID is the long UUID assigned to the feeder (see /var/cache/piaware/feeder_id or piaware.log)
(this is a bit awkward and putting together a tool to automate it is somewhere on my todo list)