How to Set AutoReply on Shared Mailbox
Go to https://outlook.office365.com – if you are not automatically signed in, login using your credentials.
In the top right corner, click on your profile icon – this will either be you profile photo or a circle with your initials.

Click ‘Open another mailbox’. A pop-up will then appear. Enter the name or email address for the shared mailbox.
The mailbox will open in a separate tab. Click on the settings cog icon on the top right toolbar. At the bottom of the settings pop-out menu, click ‘View all Outlook settings’.

Make sure you are in the Email menu to the left of the pop-up window. Then click ‘Automatic replies’ on the secondary menu. Here you can switch on automatic replies, set time frames, and write internal and external automatic replies.

When you are ready, hit save.