Hello,
The custom fields for tickets do not currently support conditional logic like this – sorry for the bad news there! You could get this set up via a customization. Our themer’s guide is the place to start if you’d like to give it a shot, and we also have a list of recommended developers that you could contact if you would like to get help with the code.
If this is a feature that you would like to see added in the future, please do let us know over in our Feature Ideas Forum! This is what we use to gauge user interest in new features and help guide future development efforts.
I’m sorry that I don’t have a better answer for you at the moment, but please let me know if you have any questions!
Thanks,
Jennifer