You can add a sensor by right-clicking the Sensors tab on the feature tree.
The following sensor is modeled after the sensor that is used in the Tutorials under Design Studies > Design Optimization of a Knob.
Once you have everything in order, click the green check mark. Afterwards, you can modify the name to "FOS" or whatever you like by double-clicking the sensor name on the feature tree. Using the Design Study, the new sensor should appear as a selection under constraints.