1. Sign in to Outlook Web App at https://outlook.office365.com
2. Click more under Folders in the left side navigation. (If you already see your mailbox name in the left side navigation, skip to step 3.)
3. Right-click your mailbox name in the left side navigation, and then click Add shared folder. In the Add shared folder dialog box, start typing the name of the shared mailbox, select the mailbox, and click Add.
After adding the shared folder, you will see the mailbox in the Folder pane each time you access
your mailbox with Outlook Web App.
Sending messages from the shared mailbox in Outlook Web App
1. When replying to messages sent to the shared mailbox, your messages should be sent from the shared mailbox address automatically.
2. Click New at the top of Outlook Web App then click Email Message
3. Click the ... (ellipses) and click Show from
4. Right-click your email address and click Remove
5. Start typing the name of the shared mailbox, select the mailbox
6. Add the message recipients
7. Type your subject
8. Type your message
9. Click Send