Frederic,
unfortunately we had to re-use the voting system we use which is mainly focused on Board of Directors elections. Let me explain how it works:
1. Under candidates, you will find a list of 2 items, "yes" and "no"
2. For either yes or no to end up in your preferences list, you mainly need to click on either yes or no, you should expect to see your final choice under Preferences and the option you didn't select under Candidates
3. While you can theoretically select both and have both options added to preferences, it's preferred to only select one single option and confirm
Please let me know if that made things clearer for you, thanks!