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The following steps will add a shared mailbox that a user already has permissions to. It will add the shared mailbox folder under the user personal mailbox in OWA.
Right Click on "Folders">Select "Add shared folder"
A window will open to enter the shared mailbox name or address.
Enter the shared mailbox name or address, it will auto populate with matches select the mailbox you want to add.
Select the add option
The shared mailbox will then be added to your profile in OWA.
