Building a Flow for the BME680 what is the set message.payload for the correct sensor?

Hi Tim,
Welcome to the Forum.
You haven't give us much to go on.
For example, what is the BME680 connected to?
The pins on a Raspberry Pi or ESP32 microcontroller, etc.. ?

What language are you using to "talk" to the BME680?
A Node-RED node or Micro-Python or Arduino, etc.. ?

You might want to read this thread as it explains what a number of users do which is to use a cheap WiFi microcontroller (e.g. ESP32-C3-Super-Mini) to act as a slave to the Raspberry Pi.