adding option to add letter to first character in text box

Hi,

I have a userform that enters a serial number in a style based on the format of the seerial on the unit.. so it might be 123-456 or 00000-00000 this is working fine and i have added an additional type, Only needs a letter B or S in front ie B-123456-1233445. If i wanted to add a letter to the start, what would i do to change the code. i have tried adding a letter to the code, missing something but not sure what. I just need either any letter or either S or B I have highlighted the lines i think i need to change in red.

cheers mark

Private Sub ComboBox1_Change()
    If ComboBox1.Value = ("Transmission Unit LL") Then Me.TBserial = ("00000000000-00000000")
    If ComboBox1.Value = ("Transmission Unit GPRS") Then Me.TBserial = ("00000000000-00000000")
    If ComboBox1.Value = ("Base Unit") Then Me.TBserial = ("00000000000-00000000")
    If ComboBox1.Value = ("Display Units") Then Me.TBserial = ("B-00000000000-00000000")
    If ComboBox1.Value = ("ComBox PSTN") Then Me.TBserial = ("000-000-000000")
    If ComboBox1.Value = ("ComBox GSM") Then Me.TBserial = ("000-000-000000")
    If ComboBox1.Value = ("Dect Meter") Then Me.TBserial = ("000-000-000000")
    
    Me.TBserial.SetFocus
    Me.TBserial.SelStart = 0
End Sub

Private Sub TBserial_KeyDown(ByVal KeyCode As MSForms.ReturnInteger, ByVal Shift As Integer)
    Dim a As Integer
    Select Case KeyCode
        Case 48 To 57
            a = TBserial.SelStart
            Select Case ComboBox1.Value
                Case "Transmission Unit LL", "Transmission Unit GPRS", "Base Unit"
                    If a = Len("00000000000-00000000") Then KeyCode = 0
                Case "Display Units"
                    If a = Len("B-00000000000-00000000") Then KeyCode = 0
                Case "ComBox PSTN", "ComBox GSM", "Dect Meter"
                    If a = Len("000-000-000000") Then KeyCode = 0
            End Select
            
            TBserial.Text = Left(TBserial.Text, a) & Chr(KeyCode) & Mid(TBserial.Text, a + 2)
            a = a + 1
            TBserial.SelStart = a
            If Mid(TBserial.Text, a + 1, 1) = "-" Then
                TBserial.SelStart = a + 1
            End If
            
            KeyCode = 0
        Case 9, 10, 13
            
        Case Else
            KeyCode = 0
    End Select
End Sub