Will Xbox charge me if I add a payment method to redeem a prepaid code?

No — adding a payment method to redeem a prepaid Game Pass code does not immediately charge you. Microsoft only charges when your prepaid subscription period ends and auto-renewal triggers. To avoid any charge, either remove the card from your account after redeeming at https://account.microsoft.com/billing/payments, or turn off recurring billing at https://account.microsoft.com/services → Manage → Turn off recurring billing.