Hey @JarvisMeier there is a macOS profile payload called LoginItemsManagedItems that I see Systems Manager does not support in the SM > Settings page yet. I'll add that in as a feature request so Systems Manager can look to add that to the UI in the future, thank you for that feedback!
In the meantime, you should still be able to add LoginItemsManagedItems to macOS devices via a custom .mobileconfig with Systems Manager. Just configure the Path of the app and Hide from User&Groups setting you need, something like this:
<key>AutoLaunchedApplicationDictionary-managed</key>
<array>
<dict>
<key>Path</key>
<string>/System/Applications/Example.app</string>
<key>Hide</key>
<false/>
</dict>
</array>
Checkout the full profile example in the Apple developer documentation. Create the .mobileconfig how you need and then upload and install through SM. Here's the steps for that part: https://documentation.meraki.com/SM/Profiles_and_Settings/Using_Custom_Apple_Profiles_with_Systems_M...
SM should be able to add this into the SM > Settings page in the future, but for now use this custom .mobileconfig route to accomplish the same thing. Good luck!