Requests the key secret from the application.
func onKeySecretNeeded(keyId: String, keySecret: inout String)
Subscribe to this event to pass the key secret (a pre-shared request authentication code) to the signing component when it is needed. The authentication combination consists of the KeyId, a non-secret unique key identifier, and the KeySecret, shared by the parties, which should be kept private. This event is an alternative for KeySecret property. Use it when you expect to process requests from requestors with different KeyIds and secrets. If you only expect to receive requests from a single requestor with a known KeyId, providing the key secret via KeyId and KeySecret properties would be an easier route.