You can find the option to add custom questions for your sessions in your session settings.
To access: you can go to your Calendar page, then click on the small gear icon on the upper-right-hand side of the session card. Then click on Edit Session.
You can then go to the section Additional options where you'll see the section Booking questions.
The first 2 questions are default and required - you'll be able to add up to 3 custom questions for your session.
