How to edit pre-session form(s) for client scheduling?

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Where can I find the source to edit the form (pictured) in order to add a fillable field for the client to complete when booking an appointment?  Currently, there is a ‘client pre-session notes’ prompt for the client to indicate what issues or concerns they want to discuss during their forthcoming Regular Client Session (55 minute) appointment but there is no available space below the prompt for any text to be entered in response. There is only a button to ‘complete appointment.’

There is an exiting list of [forms] from which I can choose when editing the Regular Client Session appointment type in the Appt. Type  Admin. area (30 min. Chat and Client Pre-Session Notes) — see highlighted image/the one at issue).  However,  I cannot see any way to access or to edit those forms—only to select or de-select them from the list for inclusion on the client side. Only the pre-existing forms are available.
 

I would like to edit the existing Client Pre-Session Notes form to add the field for clients to be able to type a reply to the prompt when booking. Regular Client Session. Seems simple. Thank you in advance to anyone who can provide some direction.