In the code, search for this function:
CAYENNE_OUT_DEFAULT()
{
// Write data to Cayenne here. This example just sends the current uptime in milliseconds on virtual channel 0.
Cayenne.virtualWrite(0, millis());
// Some examples of other functions you can use to send data.
//Cayenne.celsiusWrite(1, 22.0);
//Cayenne.luxWrite(2, 700);
//Cayenne.virtualWrite(3, 50, TYPE_PROXIMITY, UNIT_CENTIMETER);
}
this function publishes data (millis()) to cayenne on channel number 0.
Have a look at this topic to get started with cayenne Adding a New Device using MQTT and also go the through entire docs Cayenne Docs