Assuming you are using the RPi as a Blynk Client and not hosting a Local Server… then how to program your client sketch depends on the programming language you use for your Blynk Client in the RPi (NodeJS, Python, WiringPi)
You will have to examine examples for each Library type to see how they utilise Blynk, as the primary Blynk Sketch Builder examples are for Arduino/ESP, not RPi