Create instance and show your Login Form in your start up Form contractor. If the result wasn't successful so exit the Application or you can put it in a loop for let user to try 3 times. I write you simple one, you can write another yourself.
static class Program
{
[STAThread]
static void Main()
{
System.Windows.Forms.Application.Run(new Form1());
}
}
public partial class Form1: Form
{
public Form1()
{
loginForm _form = new loginForm();
if (_form.ShowDialog() == DialogResult.Cancel)
Application.Exit()
InitializeComponent();
}
}