I think this should work (I'll double check once posted)
1)Type into File Explorer 'C:\ProgramData\Microsoft\Windows\Start Menu\Programs\StartUp'
2) In Search type 'Outlook'
3) On the 'Outlook desktop app' result Right Click and select 'Open File Location'
4) File Explorer will open the location where you'll see the shortcut 'Outlook 2013'. Right Click and select 'Copy'
5) Then you will need to Paste the Outlook shortcut into the StartUp folder as above.
6) Don't forget to run Outlook minimised by right clicking the system tray icon, otherwise it will start full screen.
Confirm it works on my system.