in node/app.js [183:202]
function receivedAuthentication(event) {
var senderID = event.sender.id;
var recipientID = event.recipient.id;
var timeOfAuth = event.timestamp;
// The 'ref' field is set in the 'Send to Messenger' plugin, in the 'data-ref'
// The developer can set this to an arbitrary value to associate the
// authentication callback with the 'Send to Messenger' click event. This is
// a way to do account linking when the user clicks the 'Send to Messenger'
// plugin.
var passThroughParam = event.optin.ref;
console.log("Received authentication for user %d and page %d with pass " +
"through param '%s' at %d", senderID, recipientID, passThroughParam,
timeOfAuth);
// When an authentication is received, we'll send a message back to the sender
// to let them know it was successful.
sendTextMessage(senderID, "Authentication successful");
}