There is an example in the default skin. But, if you're starting from scratch...
Momentary isn't a 'option' that you can select per se.
What you do is use both the downclick and upclick properties. for downclick, set the controlled parameter (output in this case) to 'on' and then set the upclick to 'off'. You can mess with the duration by adjusting the minimum timings.
You can control a variable sinilarly with vote opinions of 'true'/'false' or '+1'/'+0' respectively.