if you are a talking about https://www.treasurydirect.gov/ then yes it is a legitimate site. It is a US government site run by the US department of the treasury.
Brought to you by...
TreasuryDirect is brought to you by the U.S. Department of the
Treasury Bureau of the Fiscal Service. The mission of the Fiscal
Service is to borrow the money needed to operate the federal
government and to account for the resulting debt. We do this by
offering you a variety of savings and investment products.
I entered a long list of bonds a few years ago into the system. I don't think it was very hard to do.