Help with messages

To RT List,

I am having trouble with a "Mandatory" Custom Field. RT requires that 

the field be filled in when a ticket is created. I really just want it
filled out before it is allowed to be “resolved”. I can’t think of any
way to do this other than removing the “Mandatory” requirement and add a
script that re-sets the Ticket Status value back to old value if the
Custom Field is blank when they try to “resolve” the ticket. If I do
that, I need to display some sort of error message and THAT is where I’m
stuck. I do not know how to send/create an error message that will
display like any other RT error message. Anyone ever done this before
and can help? Thanks.