How to enter a Custom field when ‘Deny Booked by, Passenger and Additional contact info to be added to Masters’ is selected?

Go Masters - Customers - Click on a customer listing - click on Custom Fields - Add Field/Open an existing field - Select Show in the Add Booking section - Tick the checkbox for Is field always editable - SAVE

By doing this you will be able to enter custom information while creating a booking even with the Deny Booked by, Passenger and Additional contact info to be added to Masters restriction enabled.