Hi @Nicklaslb,
Is it a requirement to have the data displayed in QGroundControl? I’ve previously written a post about adding a sensor to the MAVLink stream, which went through the relevant steps, although it assumes you’re using our old Companion software. If you’re using BlueOS the startup process will be different (and ideally would be handled as an extension rather than a standalone python script).
The EZO™ Conductivity Circuit you’re using seems to have support for USB, I2C, or UART communication. I expect USB will be simplest if that’s available. You should be able to use the Raspberry Pi example code provided in the documents on the product page 
Feel free to follow up with additional questions, although if you do so please provide some more context on the hardware and software versions you’re running / planning to use, and what your usage requirements are (along with “nice to have” features that aren’t strictly required).