Updating CustomField in reply


I am new to RT and its concepts. I did read most of the mail archives in the
hope of finding my answer, but no luck. I have also spend quite a lot of
time on fsck.com/rtfm/.

I have added a Global CustomField to my RT system.

The field is called: Domainname

I would like the operators to be forced to complete this field once thay
have clicked on the Reply/Resolve link. Unfortunately I have not been able
to find a way to display the CustomField input in the Reply/Resolve screen.
Can this be done? If it can be done, how would I go about doing it?

Thank you in advance.