Where to store this key — and how?

const cryptoKey =
  await crypto.subtle.generateKey({
    name: 'HMAC', hash: 'SHA-512'
  }, true, ['sign', 'verify'])

const cryptoKey =
  await crypto.subtle.generateKey({
    name: 'HMAC', hash: 'SHA-512'
  }, true, ['sign', 'verify'])