In order to add a tiebreaker question, you have 2 possibilities:
1. In the "Question" step
Once you add a question, you must select "Numeric field"
You have then the option to choose whether it's a classic question or a tiebreaker question.
2. In the "Form" step
When you create a new field, you must select "Numeric field" and tick "tiebreaker".
Warning: Remember that in the statistics of your campaign, you can select the winner among all participants by proximity to the following tiebreaker answer.