Required Combo Box field with User option for custom text

I'm using Acrobat XI Professional

My Problem:

I created a combo box with several item choices, and the option for users to enter custom text.  I would like this combo box to be REQUIRED, however, since I have the option checked for user custom text, and a combo box uses the item in a list select as a DEFAULT, it recognizes that an item was choosen and the form submits without any changes selected.

My Workaround:

Currently I have the default item as follows:  "...select from list or type specific trip type".  I also created a tooltip with instructions.  But it doesn't help if my users will ignore these notes and the form is able to submit without changing the default.

My Question: 

Is there a javascript or custom keystroke script that I can add to this combo box to throw an error when the DEFAULT item was not changed in the field?