The 300°C Thermistor works natively with Marlin, Duet/RepRapFirmware, and Klipper. Marlin
If you are compiling Marlin for use with the 300 °C Thermistor, make sure to choose the "100K thermistor - ATC Semitec 104NT-4-R025H42G". This is thermistor #5 in Marlin.
Duet/RepRapFirmware
If you are using the 300 °C Thermistor with Duet/RepRapFirmware, you will need to create a custom thermistor preset using the configurator. Click the R25 value for the correct heater and there will be a pop-up. Open the thermistor preset drop-down menu and select custom. Here you will input the following temperatures and resistances to obtain the correct configuration parameters.
T1 = 25 R1 = 100000
T2 = 85 R2 = 9094
T3 = 300 R3 = 82.78
If you are not using the configurator and simply editing an existing configuration file, use the following values in your M305 or M308 command.
T100000 B4680 C6.455513e-8
Klipper
Use the following code when configuring your "printer.cfg" file.
sensor_type: ATC Semitec 104GT-2