Sideload Outlook add-ins for testing
You can use sideloading to install an Outlook add-in for testing without having to first put it in an add-in catalog.
Note You can also sideload add-ins for other Office suite applications. For more information, see Sideload Office Add-ins for testing.
To sideload Outlook add-ins
Go to Outlook Web App.
Choose the gear icon in the top-right section and select Manage add-ins.
On the Manage add-ins page, select the + icon and then select Add from a file.
Locate the manifest file for your custom add-in and install it. Accept all prompts during the installation.
When that's done, select an existing email and you will see a horizontal bar below the email header that includes the add-in. Next, create an email, and then choose the Add-ins (or Apps) menu item to get the task pane to appear. Select the add-in and see it appear in the task pane.