enter key press event when type username and password .

        private void textBox1_KeyDown(object sender, KeyEventArgs e)

        {

            if (e.KeyCode == Keys.Enter)

            {

            }

        }

For one time Enter You can Have a Flag Variable such as ISValid = true, at first Enter change the value to ISValid = False. and check Condition in KeyDown Event as

If(ISValid)
{
//Then Do Action
}