1. Select File - > Add account
2. Enter the shared mailbox address:
3. You will be redirected to another screen asking for an email address, which will have the shared email address pre-filled in. Click on the X on the right and enter your own email address.
4. Proceed with signing in with your primary email credentials and validate with the MFA on your account, then restart Outlook.
PLEASE ALLOW ABOUT AN HOUR BEFORE TRYING TO USE THE SHARED MAILBOX
It is VERY common for outlook to freeze up at this stage when trying to use the shared mailbox. Just wait an hour then try again. If you’re still having issues please reach out to IT support.
5. Now you will need to set the shared mailbox as the default in account settings. Select File -> Account settings -> Account settings. Click on the shared mailbox and select ‘Set Default’ from the top.
6. Then, in File -> Options -> Mail, scroll down until you find the option to ‘Send from default mailbox by default’ and check box that setting.
6. If you’d like to set a signature for the shared mailbox, navigate to Insert -> Signatures -> Signatures. Choose the shared mailbox then select new and enter the signature in the body of the 'Edit Signature' section. You can set defaults for new messages and replies here.