Screen customization: How to display a default value for a drop down list ?

I'd like to display in a mandatory field a string that displays a message to user such as: "Select Responsible".

"Select Responsible" is not an element of the list of course. Moreover, upon saving the ticket, if the field is mandatory and that "Select Responsible" is still present, an error should pop up to warn the user the field is mandatory.

At the moment, when saving the ticket with such a value, the user gets logged out automatically (bug reported in the "Submit Issue" section).

Maybe I'm doing things wrong:
This is what I typed in the default value of the field:

###EVAL### $value='Select Responsible';