In Settings>Backoffice Settings, click on Custom Fields.
Click on add a field. A right side modal will appear.
1. Choose the backoffice label, and attach this field to a booker. You can also select to display this field as a text input or an option list.
2. Choose in which modules you want this field to be visible.
3. If you make the custom field visible in the Customer Portal, you can choose a Form Label (Visible to the customer) and make it optional or required.
After creating a custom field to a booking, you can view them on the bookings' page: