How do I set up out of office assistant on a secondary mailbox?

I have access to 2 mail boxes under my Outlook account. I have my own
personal mail box and then access to a shared HelpDesk mailbox. How do I set
up an out of office aasistant on the secondary mailbox only? When ever I go
through the normal set up in Outlook for the out of office assistant it will
only set up for my primary personal mail box. I can not set up a message on
the secondary mailbox at all. Can this be done?