As a rule of thumb, you should never paste your private key into websites. To generate your keys and handle your keys, use Alby or nos2x extension.

How to use Nostr with the Alby extension

https://blog.getalby.com/how-to-use-nostr-with-the-alby-extension/

The nos2x browser extension

https://youtu.be/IoLw-3ok3_M

List of extensions with window.nostr capacity for web browsers (NIP-07)

https://github.com/nostr-protocol/nips/blob/master/07.md

Use one of these to login to a web nostr client (in browser)

horse (Chrome and derivatives)

https://github.com/fiatjaf/horse

nos2x (Chrome and derivatives)

https://github.com/fiatjaf/nos2x

Alby (Chrome and derivatives, Firefox)

https://getalby.com

Blockcore (Chrome and derivatives)

https://www.blockcore.net/wallet

nos2x-fox (Firefox)

https://diegogurpegui.com/nos2x-fox/

Flamingo (Chrome and derivatives)

https://www.getflamingo.org/

AKA Profiles (Chrome, stores multiple keys)

https://github.com/neilck/aka-extension

TokenPocket (Android, IOS, Chrome and derivatives)

https://www.tokenpocket.pro/

Nostrmo (Android, IOS)

https://github.com/haorendashu/nostrmo_faq#download