After logging into your web-side account, please click the 'Options' button:   

1.   Click 'Create  A New Auto Responder:     

2.   Select format for Responder:          OR     
      Note:  In order to create an HTML Auto-Responder, you will need to have the ability to compose in HTML in the preferences.

3.   Title and create the Auto-Response as you would like.

4.   Make sure that you check the 'Enable This Auto Reply' box in the top right corner of the response.

5.   Click 'Save'.

As long as you have an Auto-Responder enabled, the message you have chosen will be the reply sent when messages are received.  You may create more than one auto-responder and enable the one you wish to use when you are out of the office.

Additionally, you may create HTML and Text Auto-Responders. . . . just make sure that you enable the Responder you wish be use.