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#1
Why not? It's simple to open Task Manager, just right-click on the taskbar and select it.
You need to be an administrator (or to run Task Manager as an administrator) to be able to disable anything in Start-up. Any other way of removing the icon would also require admin rights, such as running this in a command prompt:
taskkill /im MSASCuiL.exe
Without admin rights, your only alternative is to move the Defender icon into the 'Hidden icons', only visible when you click the up-arrow in the notification area.
Hide or Show Notification Area Icons on Taskbar in Windows 10