Catalog Items are not linked to forms. This means that you need to recreate the client side logic in a Client Script on the Catalog Item. If you just copied the code, ensure that the field on the Catalog Item is also named "service_request"

To test where you get stuck (if it doesn't work right away): add a logs (or breakpoints) to your script include to see if it is triggered, and if the code is executed as expected. Also alert the response you receive. an alert that results in an "undefined" message typically means that something broke under way.

On your original question:

if you use ...getXMLAnswer(callBackFunction), then you don't need the "response.responseXML.doc..." etc. Your response simply contains the string(!) with the response from the Server.

if you use getXML(callbackfunction), then indeed you need to extract the answer thourgh those steps.

good luck!