Related Questions:
Customize my intake form
Customize the fields on my website
Can I add custom fields to my intake form?
Procedure:
Select Settings
Click Tools -> Intake Forms
Hover over the Intake form & Click Edit
On the Fields tab, Select Add Element
Select Input Type field
Under Field, Select the relevant Booking Custom Field
Update the label if necessary (this is visible to the guest)
Click Save
Note: Only Booking, Contact & Account custom fields can be used in Intake Forms.