Go to Configuration->Job Setting -> Custom Field. You can create custom fields with Text, Drop down, check box or radio button type.
The job custom fields are available in job details page in web, mobile app and also Instant Booking Page setup.
Job Detail
You can choose to enable them by Show toggle and make it required by the Mandatory toggle.
This is how the custom field will be presented to your customer in the online booking form.
Mobile App
