1. A computer-implemented method to use discretionary data fields to secure transactions, comprising: storing, by one or more computing devices, a payment account identifier that identifies a payment account associated with an account of a user that is issued by a payment processing system, the payment account identifier identifying the payment account and being arranged in a plurality of fields and comprising a discretionary data field; receiving, by the one or more computing devices, a payment request in a communication from a merchant computing system, the payment request comprising a request for payment account information for a payment account to use to fund a payment transaction; at a time after receiving the payment request, notifying, by the one or more computing devices, the payment processing system that a transaction is pending, the notifying being in a separate communication from the communication with the merchant computing system; receiving, by the one or more computing devices, an authentication challenge from the payment processing system, the authentication challenge being communicated from the payment processing system upon receipt of the notification; obtaining, by the one or more computing devices, the payment account identifier and a challenge response to the authentication challenge; embedding, by the one or more computing devices, the challenge response in the discretionary data field associated with the payment account identifier; and communicating, by the one or more computing devices, the payment account identifier including the discretionary data field having embedded therein the challenge response to the merchant computing system to be communicated from the merchant computing system to the payment processing system to authorize the payment transaction based on a match between the challenge response and a challenge response stored on the payment processing system.