For my sysadmins brothers and sisters out there, you’ll eventually get a request to manage that start-up status for Windows Store Apps (UWP) for your users.
You’ll find the relevant setting here: HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppModel\SystemAppData\[KEY FOR THIS APP]\SlackStartup\
There you’ll find a DWORD called “State” which can have these values:
State=1, disabled by user
Just push out your desired value with a GPO!