To set up automatic replies for a shared mailbox in webmail, follow these steps:
- The user with Full Access permission signs in to Outlook on the web
- Click on your initials
- Click Open another mailbox

- Enter the shared mailbox
- Click Open

- Click Settings

- Click Mail
- Click Automatic replies
- Select Automatic replies on
- Specify the message (inside your organization)

- Select Send replies outside your organization
- Specify the message (outside your organization)
- Click Save

You have successfully set up automatic reply for the shared mailbox in Outlook on the web