Private Sub cmdAdd_Click()
Dim fnum As Integer
Dim txt As String

    ' Open the file for append.
    fnum = FreeFile
    Open "C:\autoexec.bat" For Append As fnum
    
    ' Add the command.
    txt = txtCommand.Text
    Print #fnum, txt
    
    ' Close the file.
    Close fnum
    
    MsgBox "Ok"
End Sub