Out of Office (Shared Mailbox)
1. Log into the Outlook Web App at https://outlook.office365.com using your own Office 365 credentials
2. Click on your profile image (or placeholder image) on the top right
3. Choose Open another mailbox
4. Type in the name or email address of the Shared Mailbox and select it
5. Click the settings cog on the top right and then click Automatic replies
6. Change it to "Send automatic replies," type your message, and click OK.
From: https://gcits.com/knowledge-base/easily-set-automatic-replies-office-shared-mailboxes-office-365/