Despite the name, "Startup" items do not run when Windows starts – they run when the user logs in (specifically when their initial Explorer.exe process starts). So items in a user's personal "Startup" folder will only be started when that specific user logs in. If another user logs in (or nobody does), then those items won't be started at all.
If this computer is dedicated to that app (and if there is no way to avoid it being a desktop app), you can set up automatic logon.
Ideally, however, programs that must be running 24/7 should not be desktop apps; they should be written as system services (without any user interface – if desktop UI is needed, it is best to design it as a separate app that connects to the service and controls it).