To install the Outlook add-in for mobile, you must go to your browser version of Outlook and login. Once installed here it will begin to appear on all your Outlook apps on mobile devices.
1 . Login to your Outlook account at https://outlook.office.com/owa/
2 . In the top right, click on Settings → Manage Add-Ins
3 . Select My add-ins → Add custom add-in → From URL
5 . Restart web/desktop/mobile app