AFAIK you can't hide a process from the Task Manager, and that's a good thing. Preventing a user from seeing and ending an active process is plain and simply wrong. Can you imagine being an administrator and yet not being able to end a process on your machine? :)
As for the ShowInTaskBar
property, it only determines if the form is visible as a Taskbar button or not. Not related to Task Manager in any way.
ANSWER TO YOUR COMMENT BELOW:
If you are using Winforms then in your form's Closing
event there's a CloseReason
property that can inform you about the app being closed from the Task Manager. Unfortunately, you will find out that it ain't 100% reliable.