Perform function on numbers?

If the number will always be exactly 6 digits, then you can do it more specifically with:

One way
  public Form1()
  {
  InitializeComponent();
  }

  private void Form1_Load(object sender, EventArgs e)
  {

  }

  private void button1_Click(object sender, EventArgs e)
  {
  if (string.IsNullOrEmpty(textBox1.Text))
  {
  return;
  }
  string ss = textBox1.Text;
  textBox2.Text = StringExtensions.MultiInsert(ss, "-", 1, 3);
  }



  }
}

public static class StringExtensions
{
  public static string MultiInsert(string str, string insertChar, params int[] positions)
  {
  StringBuilder sb = new StringBuilder(str.Length + (positions.Length * insertChar.Length));
  var posLookup = new HashSet(positions);
  for (int i = 0; i < str.Length; i++)
  {
  sb.Append(str[i]);
  if (posLookup.Contains(i))
  sb.Append(insertChar);

  }
  return sb.ToString();

  }