cablop on "[Plugin: Contact Form 7] How to Enable Text field if option is not present in dropdown select?"

We need to create a dropdown select with some cities, if the city is not present in the options, then a text box is enabled for the user to manually type it, if the option is present that text input should be not visible.

How can we accomplish that in CF7?

Thanks in advance.

http://wordpress.org/plugins/contact-form-7/