Try using SendKeys.SendWait instead of Send.
private void button1_Click(object sender, EventArgs e)
{
IntPtr handle = FindWindow("L2UnrealWWindowsViewportWindow", "Lineage II");
if (!handle.Equals(IntPtr.Zero))
{
if (SetForegroundWindow(handle))
SendKeys.SendWait(Username.Text);
SendKeys.SendWait("{TAB}");
SendKeys.SendWait(Password.Text);
}
}