Allowing user to provide a required CF at resolve time

We have a CF named ‘ServiceTag’ and we want to ensure a value is provided before a ticket can be resolved.

Here’s what we already have working:

  1.   We created a Callback on Update.html which checks whether the user is a. trying to resolve and b. has provided a service tag.  If not, we do not allow the resolve to proceed, and the user gets the message "Sorry, can't resolve ticket without a service tag." at the top of the form.
  2.   We've copied Update.html to local/ and added a Service Tag form field on the page.  If the service tag is already defined, this field populates the value correctly.

So far so good. What I haven’t quite figured out how to do is to capture the new value if the user provides one.

Of course, the common case we are trying to handle is when the user attempts to resolve a ticket that has no service tag. So they click Resolve on the ticket, type in some message, and then they click Update Ticket the first time and get the message “Sorry, can’t resolve ticket without a service tag.” We now want to allow them to simply provide the service tag on the form, and click Update Ticket again, and then we want to store the service tag and allow the resolve to proceed.

I’m guessing I need to do something in Update.html’s INIT area to detect whether the ServiceTag value has changed, but I am not quite sure how to proceed. Can anyone provide some pointers?