Hello. Unless you create a custom form, I am unaware of anyway to directly access that field like you would with an editor element you added through $w…
I have not done much testing with the Wix bookings API myself yet, but would you be able to use checkoutBooking ?
Also, if you haven’t run into it yet here is an article about the bookings lifecycle that might be of use in figuring out where/when to query the collection/update data etc.
If you really need full control of the fields, you will want to build a custom bookings experience/pages which it sounds like you already know about but are trying to avoid. I am just mentioning it for anyone passing through this post.