This example from the help for "OpenTextFile method" may help you

Sub OpenTextFileTest
Const ForReading = 1, ForWriting = 2, ForAppending = 3
Dim fs, f
Set fs = CreateObject("Scripting.FileSystemObject")
Set f = fs.OpenTextFile("c:\testfile.txt", ForAppending,TristateFalse)
f.Write "Hello world!"
f.Close
End Sub