C# Signature:

[DllImport("user32.dll")]
static extern bool SetClipboardData(uint uFormat, IntPtr data);

Code Sample

OpenClipboard(IntPtr.Zero);
var yourString = "Hello World!";
var ptr = Marshal.StringToHGlobalUni(yourString);
SetClipboardData(13, ptr);
CloseClipboard();
Marshal.FreeHGlobal(ptr);