Related article: Send to Case - Overview
The Send to Case Gardiant add-in is available in the web Outlook but it needs to be pinned in order for the add-in to show on each email without additional action.
In order to pin the Send to Case, log in to the web version of Outlook.
From the Settings (gear) icon, click on View all Outlook settings at the bottom of that screen:
Go through Mail > Customize actions > check Send to Case under the Message surface section:
Next time you select an email, the Send to Case icon will be present to use:
