To use that site you do need to login with your nostr key. 

Generally, it's safer to user a signing extension though, instead of pasting you nsec in to random sites. 

You can read about which extensions to use here: https://orangepill.dev/nostr-guides/guide-nostr-key-generation-and-management/#nos2x-nos2x-fox-and-alby-nostr-signer-extensions