OrcaSlicer use M141/M191 command to control active chamber heater.

If Activate temperature control is checked, OrcaSlicer will insert M191 command at the beginning of the gcode(before Machine G-code).

Note: If the machine is equipped with an auxiliary fan, OrcaSlicer will automatically activate the fan during the heating period to help circulate air in the chamber.

There are two chamber temperature variables available that we can use in Machine G-code to control the chamber temperature, if you prefer:
To access the chamber temperature set in the first filament, use:
M191 S{chamber_temperature[0]}
To use the overall chamber temperature, which is the highest chamber temperature set across all filaments, use:
M191 S{overall_chamber_temperature}

--------------------------Klipper--------------------------
If you are using Klipper, you can define these macros to control the active chamber heater.
Bellow is a reference configuration for Klipper.
Note: Don't forget to change the pin name/values to the actual values you are using in the configuration

[heater_generic chamber_heater]
heater_pin:PB10
max_power:1.0
# Orca note: here the temperature sensor should be the sensor you are using for chamber temperature, not the PTC sensor
sensor_type:NTC 100K MGB18-104F39050L32
sensor_pin:PA1
control = pid
pid_Kp = 63.418 
pid_ki = 0.960
pid_kd = 1244.716
min_temp:0
max_temp:70

[gcode_macro M141]
gcode:
    SET_HEATER_TEMPERATURE HEATER=chamber_heater TARGET={params.S|default(0)}

[gcode_macro M191]
gcode:
    {% set s = params.S|float %}
    {% if s == 0 %}
        # If target temperature is 0, do nothing
        M117 Chamber heating cancelled
    {% else %}
        SET_HEATER_TEMPERATURE HEATER=chamber_heater TARGET={s}
        # Orca: uncomment the following line if you want to use heat bed to assist chamber heating
        # M140 S100
        TEMPERATURE_WAIT SENSOR="heater_generic chamber_heater" MINIMUM={s-1} MAXIMUM={s+1}
        M117 Chamber at target temperature
    {% endif %}