How can i create a window that stays topmost and don't loose focus with ALT+TAB or by any other process lunch like Task Manager?

I can create a fullscreen topmost window but unable to keep it topmost. For example, ALT+TAB brings up the taskbar and if I start Task Manager, the Task Manager window becomes the topmost window. I want to prevent loosing focus of my fullscreen window and don't want any other window to pop up.

Thanks.