In order to do this you will need Full Access to the user or shared mailbox in question. Please request this first.
To set the automatic replies:
- Go to https://outlook.office.com/mail/
- Click your profile picture / initials in the top right corner and choose “Open Another Mailbox”
- Search for the user or mailbox and click “Open”
- Go to Settings and search “Automatic replies”
- Configure the automatic reply as required and save the changes.