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/Contact/Account 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.