Cannot toggle fullscreen at game start

I find many way on answerhub/forum to do this, but I cannot found any way to do this.

I want to let my game start with fullscreen default, and play startup movie, final show my game.
But even I user command-line argument “-FULLSCREEN” or “GameUserSettings.ini” and direct call C++ code to do this, the startup movie still play in windowed mode.

And no one answer this correctly.
Is this not support now, or I didn’t use the correctly way to enable it?