How to set PopupWindow

void UMyBlueprintFunctionLibrary::setWindowPop()
{
UGameEngine* GameEngine = Cast(GEngine);
if (GameEngine != nullptr)
{
TSharedPtr GameViewportWindow = GameEngine->GameViewportWindow.Pin();
if (GameViewportWindow.IsValid())
{
TSharedPtr GW = GameViewportWindow->GetNativeWindow();
FGenericWindowDefinition WD = GW->GetDefinition();
WD.IsRegularWindow = false;
}
}
}

I want to set a GameWindow with no border.But not full screen.

FGenericWindowDefinition::IsRegularWindow = false; or SWindow::IsPopupWindow = true; ?
How to set them up? How do I do that?

Thanks!!!