It is currently not possible to use a Hardware Security Key for trading 2FA.
IMPORTANT: After enabling 2FA for trading, you must also enable the Global Settings Lock (GSL) or a sign-in 2FA in order for it to be effective. Enabling sign-in 2FA activates Step-up 2FA on your account. Step-up 2FA is an extra step that is required anytime you want to add, edit or remove any 2FA settings on your account. Once a change to your Kraken account is requested, you will be prompted to confirm your sign-in 2FA again via the 6-digit code from your authenticator app or via your Hardware Security Key. The code must be entered correctly before any changes can be made to the 2FA settings on your account. If you choose not to use sign-in 2FA, you must enable the GSL. Without the GSL, the trading 2FA can be easily removed or changed by anyone who gains access to the account.