You can add a restart button in any place you can add custom tags, for example the Custom Thank you Message, Outcomes or Grades. To add your restart button just add the text {restart button} to your message and a button will display that says 'Start Again'.
Here is a guide to Can I add a button to retake / start the quiz again?:
https://www.quiz-maker.com/Quiz-Help/FAQ-Restart-Button