There are third party sites popping up, charging as much as $25 per aircraft. The real site is here https://registermyuas.faa.gov/. It's an initial one ttime only 5 dollar charge(Refunded until Junuary), you get a number that covers all of your systems that you use personally for recreational hobby use. It does not meet the 333 requirements for commercial use. Aircraft that you plan to fly under your exemption will still need to use the paper process Form 8050-1 and get an N-Number.