Certain layers, including Humidex, aren’t currently available on the API. A complete list of queryable fields is available in our API Documentation.
One option is to locally calculate Humidex using temperature and humidity data from the API. If doing this, please note that temperature and humidity from the API are uncorrected. You can read more about this and available corrections in the Temperature and Humidity section of our What do PurpleAir Sensors Measure article.