This is easily accomplished by using the web interface

• Log in to email using the links provided (or see the GEN Website for reminders)
• In the left menu choose Mail → Automatic Reply.
• Fill in subject and message body. Set start/end dates if available, and options such as reply frequency or send only to contacts if offered.
• Save and enable the autoresponder.

If you've set an end date, the system will automatically stop using the autoresponder on that date, otherwise you'll need to manually disable it. 

If in doubt, feel free to ask at the HelpDesk.