[DllImport("user32.dll", SetLastError = true)]

private static extern bool OpenClipboard(IntPtr hWndNewOwner);

[DllImport("user32.dll", SetLastError = true)]

private static extern bool CloseClipboard();

[DllImport("user32.dll")]

internal static extern bool SetClipboardData(uint uFormat, IntPtr data);

OpenClipboard(IntPtr.Zero);

var yourString = "Hello World!";

var ptr = Marshal.StringToHGlobalUni(yourString);

SetClipboardData(13, ptr);

CloseClipboard();

Marshal.FreeHGlobal(ptr);