The way these sites are supposed to work is through a particular State's treasurer or department of revenue. From that page you should be directed to the specific State agency's website, asked to fill out a form that you'll need to have notarized and send TO that State agency.
A good rule of thumb is if they lead you to a legitimate State Treasurer or DOR that has a mailing address you can independently verify, you're probably fine. If they try to get you to send your docs to a P.O. Box or the last known address of a Mr. Kurt Swindler, it may not be legit.