Add a field for specific category

Hi at all

i've just got an other question.
I want to add a new field (called "NPR und FAX Nummer") when user chose the category "Drucker und Fax". If he chose another category the fiel shouldn't be visible.

First I rename the field "sr.cust_text1" in the following File:
this works and i can add it to the end user portal

Then i found an Guidance to add the field only when a specific category is chosen...
i tried this in file "<installpath>\SysAidServer\root\WEB-INF\conf\html\submit-service-request.htm"

but i always see the field... where is my fault?
and maybe you know how can i make the field required...?

thank you