MSIX application, unable to stop automatic start-up on Windows 8.1

The above answer should work. Although if you can't modify the package and reinstall, it might be easier to post-install delete the startup task that was installed. https://github.com/microsoft/msix-packaging/blob/master/MsixCore/msixmgr/StartupTask.cpp describes how the startup task is parsed when installed via msixmgr, youshould be able to open task scheduler and find and disable the task. In task scheduler, navigate to folder Microsoft\Windows\MsixCore and find the relevant task to disable.