Make specific randomly generated codes with timer - vb.net

Question

I would like to make randomly generated codes when a timer is finished.
But I would like to use this code ABCDEFGHIJKLOMNOPQRSTUVWXYZ0123456789
on this format XXXX-XXXX-XXXX (replace X with a randomly generated number/letter)
And I would like to insert the code in this
Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
ProgressBar1.Increment(3)
If ProgressBar1.Value = 100 Then
TextBox1.Text = "replace textbox by code"
Timer1.Stop()
End If
End Sub

Answer 1

Here's a solution using LINQ.
Private Sub Timer1_Tick(sender As Object, e As EventArgs) Handles Timer1.Tick
ProgressBar1.Increment(3)
If ProgressBar1.Value = 100 Then
Dim chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
Dim random = New Random()
Dim num1 = New String(Enumerable.Repeat(chars, 4).Select(Function(s) s(random.Next(s.Length))).ToArray())
Dim num2 = New String(Enumerable.Repeat(chars, 4).Select(Function(s) s(random.Next(s.Length))).ToArray())
Dim num3 = New String(Enumerable.Repeat(chars, 4).Select(Function(s) s(random.Next(s.Length))).ToArray())
Dim result = num1 & "-" & num2 & "-" & num3
TextBox1.Text = result
Timer1.Stop()
End If
End Sub

Answer 2

With this, you'd just change your TextBox1.Text = to point to the getNewRandomString() function.
Private Function getNewRandomString() As String
Dim sb As New StringBuilder
For i = 1 To 3
For j = 1 To 4
sb.Append(getNewRandomLetter())
Next
sb.Append("-")
Next
Return sb.ToString.Trim("-")
End Function
Private Function getNewRandomLetter() As Char
Dim chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
Return (chars(GetRandom(0, chars.Length - 1)))
End Function
Public Function GetRandom(ByVal Min As Integer, ByVal Max As Integer) As Integer
Static Generator As System.Random = New System.Random()
Return Generator.Next(Min, Max + 1)
End Function