You'll need to use a signing extension to do so. if you're on iOS, you can install the Safari extension on the App Store called "Nostore".  This will allow you to sign-in to any nostr web app with your private key. Never enter your private key into an external website. if you're on Android, you can do this too, but it's a bit more complicated. You'll need to use a different web browser that allows mobile extensions, such as Kiwi Browser. Then, install the Chrome extension called nos2x. Enter your key into that and you're good to go just like above! Good luck!