Determine if individual letters are in a textbox

Hi, Im new to this and wanted to know: I have a standard exe with two text boxes and one command button, how would I code that to determine if certain letters are in one textbox so they can be converted to say a specific number in the second text box, almost like encryption. Thanks

You can use this as a starting point

Code:

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Private Sub cmdConvert_Click()

    Dim intIndex As Integer
   
    For intIndex = 1 To Len(txtInput)
        Select Case UCase(Mid$(txtInput, intIndex, 1))
            Case "A"
                txtOutput = txtOutput & "3"
            Case "B"
                txtOutput = txtOutput & "7"
            Case "C"
                txtOutput = txtOutput & "9"
            Case Else
                txtOutput = txtOutput & "?"
        End Select
    Next

End Sub

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Check out This Post for my simple encryption example, as well as some other ideas.