Bynk uses "Virtual Pins" to pass data between the App and the hardware.

The Blynk Library calls BLYNK_WRITE(Virtual Pin#) when the appropriate virtual pin value is changed by the app.

In my Firmware once the function is called I store the Value of the Vpin in the appropriate variable for use later.

As an example

BLYNK_WRITE(V3)
/*
 * sets the temperature level for the fans to come on
 */
{
  maxTemp = param.asInt();
  Serial.print ("Temp Set ");
  Serial.println ( maxTemp);
}

every time the temp Slider is moved the value of the slider is stored and printed.

Unfortunately the value stored will be lost if the NodeMCU is ever reset - ## Need to store the values in Flash to be read when the unit reboots ##