SqlConnection con = new SqlConnection("Data Source=YOUR-COMPUTER-NAME\\SQLEXPRESS;Initial Catalog=loginapp;Integrated Security=True;TrustServerCertificate=True");
con.Open();
string query = "SELECT COUNT(*) FROM loginapp WHERE username=@username AND password=@password";
SqlCommand cmd = new SqlCommand(query, con);
cmd.Parameters.AddWithValue("@username", txtUser.Text);
cmd.Parameters.AddWithValue("@password", txtPass.Text);
int count = (int)cmd.ExecuteScalar();
con.Close();
if (count > 0)
{
MessageBox.Show("login success", "info", MessageBoxButtons.OK, MessageBoxIcon.Information);
}
else
{
MessageBox.Show("error in login");
}

//password show hide:

txtPassword.UseSystemPasswordChar = !checkBox1.Checked;

//close form:

this.Close();

FULL TUTORIAL: