Post values / prepopulate bookings page

Hi, when people book an appointment via bookings we already know who they are and want to pass a customer ID number along to the bookings page. That could be accomplished by adding a variable to the booking page URL and having a custom field on it (preferably hidden). But the custom input field does not have a name tag and adding URL parameters to the page gives an error.

Short of having to rewrite the above functionality using the API, how do I accomplish the above?

Thank you.