Custom fields in book screen

I'd like to add a field to the book screen that requires the user to add a subject to their request.  At the moment, calendar entries only show, for example, "60-minute discussion with ...", so it's difficult to know in advance what sort of conversation you're getting into.  Copilot tells me there's an "add a custom field" option, but I can't find it.  Does anyone have a suggestion?  Thanks.