Get FormFields to Submit with checkoutBooking?

Question:
One of the arguments that needs to be submitted when using checkoutBooking is the FormFields collection object that is based on the Custom Booking Form for the service I want to book. I am drawing the data for the checkoutBooking from a custom Sign Up form I created myself. When looking at examples, I see that the FormField object should have a key value pair where the key is the ‘_id’ for each of the fields that the Custom Booking Form uses. I do not know how to obtain these ‘_id’ numbers. In this case, how can I successfully instantiate a FormFields object with the correct information for the service I am booking?