Add a BLEDescriptor to the characteristic.
bleCharacteristic.addDescriptor(bleDescriptor)
Nothing
// Bluetooth® Low Energy Battery Level Characteristic
BLEUnsignedCharCharacteristic batteryLevelChar("2A19", // standard 16-bit characteristic UUID
BLERead | BLENotify); // remote clients will be able to get notifications if this characteristic changes
BLEDescriptor batteryLevelDescriptor("2901", "millis");
batteryLevelChar.addDescriptor(batteryLevelDescriptor);