Here's a useful snippet for using your secret key on a website that only supports extension login (useful for testing stuff in a private window):
import('https://cdn.jsdelivr.net/npm/@welshman/signer@latest/+esm').then(mod => {
const secret = 'your hex secret key here'
const signer = mod.Nip01Signer.fromSecret(secret)
window.nostr = {
getPublicKey: signer.getPubkey,
signEvent: signer.sign,
}
})