如果用户输入的用户名无效，您似乎希望将重点放在用户名上;如果用户名有效，您似乎希望将重点放在密码上：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Check_User_Input
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            label6.Text = txt_nomeUtente.Text.Length.ToString("N0");
        }

        private void button15_Click(object sender, EventArgs e)
        {
            CheckForValidEntries();
        }

        private void CheckForValidEntries()
        {
            if (string.IsNullOrEmpty(txt_nomeUtente.Text))
            {
                label6.ForeColor = Color.Red;
                label6.Text = "username empty, cannot be left empty";
                //txt_nomeUtente.Text += ((Button)sender).Text;
            }
            else
            {
                label6.ForeColor = Color.Green;
                label6.Text = "username is acceptable";
                txt_Password.Focus();
                if (string.IsNullOrEmpty(txt_Password.Text))
                {
                    label6.ForeColor = Color.Red;
                    label6.Text = " password is empty";
                }
                else
                {
                    label6.ForeColor = Color.Green;
                    label6.Text = "La password is acceptable";
                }
            }
        }

        private void txt_nomeUtente_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter)
                CheckForValidEntries();
        }
    }
}