1. Sign in account in Outlook App
a. Download outlook app from app store. Click here
for more info.
b. Tap Get Started button
2. Tap Add Account button in left navigation pane, then tap Add Shared Mailbox.
3. Select the account that has permissions to access the shared mailbox.
4. After the account setup process completes, the shared mailbox will display in your account list in Outlook App.