I don’t have a schematic of what you are trying to do nor do I have any other information.
The heater control needs to be a PWM output some how.
The Thermistor will probably need some sort of bias circuitry but would ultimately connect to an analog input and get run through an ADC.
Your code would somehow need to convert the digital output of the ADC to something that controls the PWM output.
To give you more details I would need more Details.
Good luck,
Circuitsville