To install the add-in as a system administrator for multiple users follow the steps below.
1 . Login to below Office 365 Admin Portal URL to manage Add-Ins https://portal.office.com/adminportal/home#/Settings/ServicesAndAddIns
2 . Click on "Deploy Add-in"
3 . Select "I have a URL for manifest file" and provide the below URL and click Next URL: https://wisetime.io/ic/outlook/manifest.xml
4 . It will show information from manifest file, click Next
5 . Select appropriate option to enable it optional or mandatory and click Next
6 . Select appropriate option to enable for Everyone or list of users and click Save, then Close
7 . Once installed, you can see the add-in in the "Services and Add-Ins" section.