Add auto reply to shared mailbox
Open the shared mailbox by logging in to Office.com and going to Outlook.
Once you have the correct mailbox open, click on the cog that opens the Settings. This is located at the top right of the mailbox screen
At the bottom click on ‘View all Outlook settings’
From the settings options, select ‘Automatic Replies’
Toggle the Automatic replies to on, then add the message you want to send internal and external to the company.