You can use sideloading to install an Outlook add-in for testing without having to first put it in an add-in catalog.
Sideload an add-in in Outlook on the web
Go to Outlook on the web.
Choose the gear icon in the top-right section and select Manage integrations.

On the Manage integrations page, select Add-Ins, select the Click here to add a custom add-in link, and then select Add from file....

Locate the manifest file for your custom add-in and install it. Accept all prompts during the installation.
Sideload an add-in in Outlook on the desktop
Open Outlook 2016, Outlook 2013, or Outlook 2016 for Mac.
Select the Store button on the ribbon.

On the Store page, select Add-Ins, select the Click here to add a custom add-in link, and then select Add from file....

Locate the manifest file for your custom add-in and install it. Accept all prompts during the installation.
