Click on the link you quoted for basics. It’s an Advanced option, so goes on Options screen as an Edit.
Satisfying “requires administrative privileges” is not always a smooth path. If your Installation is now a Windows Service, you’re all set with that. You would have used Duplicati.WindowsService.exe for that.
Install Duplicati as a service is a video. Migrating from User to Service install on Windows may also fit.
If you’d rather not install as a service, then that’s the usual Windows install, and it runs after user login. Windows will only add administrative privileges after a UAC prompt, and Duplicati will run as that user, which may require database moves if current user is a Standard user and borrows another account.
Manual start icon can be made from a shortcut to Duplicati → right-click → Properties → Advanced → “Run as admistrator”. Autostart as Admin discusses the issue and links to a Task scheduler method.
Sorry it’s so hard to run as adminstrator, but Windows wants it that way. Maybe someday the Duplicati installer can hide some of this, or the manual can provide official guidance if the team can develop that.