In Web Outlook - your Thread for Email Outlook add-in will show at the top right corner on each email you open. 

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If the Thread for email icon is not visible in the top right corner, click on the App icon and select it from the pop-up window. 

If you wish to pin it, so that the add-in sidebar shows each time you open any email, click on the pin icon. 

In Desktop Outlook - your Thread for Email Outlook add-in will show in the top right side of your toolbar when you click on any email. 

If you wish to pin it, so that the add-in sidebar shows each time you open any email, click on the pin icon.