This sensor is used for passing Mavlink text messages which are displayed at the bottom of the Taranis LCD and on the Mavlink screen (when you press MENU from the flight screen). To discover the sensor 1000, a Mavlink text message must be received during the discovery process. You can force a message to be transmitted by power cycling the Pixhawk while in discovery. You can add the sensor manually too, but make sure the sensorID is the same as the others (sensorID 28). You can look at the “Taranis_settings.eepe” file, which you would open with OpenTX companion for more details on sensor setup.
Generally, it is best to start the discovery process, turn on the Pixhawk, then wait approximately 15 seconds until all sensors are discovered.