Step 1: Go to “O365 Admin Center.”
Click Settings on the left side menu and choose Add-ins
Step 2: Access deployment menu
Click Deploy Add-in and click Next
Step 3: Add the manifest URL
Select "I have a URL for the manifest file" and add the manifest URL. To find the manifest URL, click here.
Step 4: User default visibility
Click Next and set the default visibility of the add-in to Mandatory. This way it will show up on the users' Outlook/Office365 Ribbon
Step 5: User access
Choose who should have access to the add-in.