You can find this section in HostBill Admin → Settings → Products & Services → Your order page → Your product → Components → Forms → Add new form field → Custom/Extra field types → Select contact.

Basic settings

In the Basic settings section you can configure:

• Field name -  the name for this field

• Required field - choose if you want the field to be required to proceed
• Description - optionally you can add the description for the field
• Form group 

Advanced settings

In the Advanced settings section you can configure:

• CSS Class - field container will be displayed with this css class
• Group - supported by some order pages to group option
• Variable name - to use in emails
• Admin only -  if this option is enabled only admin can see this field
• Show in cart - display this field during order
• Show in Invoice - choose when to add field description to invoice line:
  • Default - show when priced or enabled in cart and not empty
  • Priced - show when selected value isn't free
  • Force - show even when it's empty or free

Sample client area view

Image Added