How to Add/Open a Shared Mailbox in Outlook

Method 1: Using Outlook on the Web

Log in: Sign in to your Outlook on the web account. https://outlook.office.com/
Open another mailbox: Click your profile picture or initials at the top right corner.
Select the shared mailbox: In the dropdown menu, choose "Open another mailbox".
Enter the mailbox address: Type the email address of the shared mailbox and click "Open".

Method 2: Adding the Shared Mailbox to Your Outlook Profile

Open Outlook: Launch the Outlook desktop application.
Go to Account Settings: Click "File" > "Account Settings" > "Account Settings".
Select your email account: In the "Email" tab, choose your primary email account.
Click "Change": Press the "Change" button.
Add the shared mailbox: In the "Change Account" window, click "More Settings" > "Advanced" > "Add".
Enter mailbox details: Type the shared mailbox's email address and display name. Click "OK" to close each window.
Restart Outlook: Close and reopen Outlook to see the shared mailbox in your folder list.

Important Note: If you are having trouble adding or getting an error it may be because it's already in your sidebar automatically.

From your main Outlook window scroll past all of your other folders and accounts and expand the account you are looking for.

More details: Open and use a shared mailbox in Outlook - Microsoft Support