Here is a little code:
When you type, add a comma. When you backspace, remove comma and last char.
Private Sub TextBox1_KeyDown(sender As Object, e As KeyEventArgs) Handles TextBox1.KeyDown
If e.KeyCode = Keys.Back Then
If TextBox1.Text.Count > 2 Then
TextBox1.Text = TextBox1.Text.Substring(0, TextBox1.Text.Length - 2)
TextBox1.Select(TextBox1.Text.Length, 0)
End If
End If
End Sub
Private Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged
If Not TextBox1.Text.Last = "," Then
TextBox1.Text = TextBox1.Text & ","
TextBox1.Select(TextBox1.Text.Length, 0)
End If
End Sub