Переопределите свойства CreateParams и ShowWithoutActivation формы, например:
protected override CreateParams CreateParams
{
get
{
CreateParams baseParams = base.CreateParams;
// WS_EX_NOACTIVATE = 0x08000000,
// WS_EX_TOOLWINDOW = 0x00000080,
baseParams.ExStyle |= ( int )(
Win32.ExtendedWindowStyles.WS_EX_NOACTIVATE |
Win32.ExtendedWindowStyles.WS_EX_TOOLWINDOW );
return baseParams;
}
}
protected override bool ShowWithoutActivation
{
get { return true; }
}