winforms login form login button click go to another form

it's not quite right to start your mdi form from login form. your login form should be called from mdi form. it can be done in several places for example:
In MDI form constructor(user wouldn't see MDIForm before login)

//MDIForm constructor
public MDIForm()
{

  using (AuthorizeForm form = new AuthorizeForm())
                {
                    if (form.ShowDialog() != DialogResult.OK) //login incorrect
                    {
                        System.Environment.Exit(1);
                        return;
                    }
                }
}

//it's your AutorizeForm
public class AutorizeForm()
{
     private void login_Click(object sender, EventArgs e)
     {
       try
       {
            System.Data.OleDb.OleDbConnection conn = new System.Data.OleDb.OleDbConnection();
            conn.ConnectionString = BillingApplication.Properties.Settings.Default.DB_BillingConnectionString;
            conn.Open();
 
            string selectString = "SELECT * FROM login WHERE (Username = '" + textBox1.Text + "' AND Password = '" + textBox2.Text + "')";
            OleDbCommand cmd = new OleDbCommand(selectString, conn);
            OleDbDataReader dr = cmd.ExecuteReader();
            if (dr.HasRows == true)
            {
                    DialogResult = DialogResult.OK;
                    return;
            }
        }
        catch (Exception)
        {
            MessageBox.Show("Error with Database Connection");
        }
        finally
        {
            if(conn != null && conn.State != == ConnectionState.Closed)
            {
                conn.Close();
            }
        }
   DialogResult = DialogResult.NO;
  }
}

why do you need MdI? if it can be done simply by opening another form and hiding Ist form.

private void login_Click(object sender, EventArgs e)
{
       try
       {
            System.Data.OleDb.OleDbConnection conn = new System.Data.OleDb.OleDbConnection();
            conn.ConnectionString = BillingApplication.Properties.Settings.Default.DB_BillingConnectionString;
            conn.Open();
 
            string selectString = "SELECT * FROM login WHERE (Username = '" + textBox1.Text + "' AND Password = '" + textBox2.Text + "')";
            OleDbCommand cmd = new OleDbCommand(selectString, conn);
            OleDbDataReader dr = cmd.ExecuteReader();
            if (dr.HasRows == true)
            {
                  
                  Form2 f2 = new Form2();
                  f2.Show();  // Display the new form.
           this.Hide(); //hide current form
            

            }
            catch (Exception)
            {
                MessageBox.Show("Error with Database Connection");
            }
}

and do what you want to do in Form2