Code to run macro on save

I have the following code in my workbook, it allows me to run the macro to
export all the data to a text file, How can I include a code into this to
export the data to a text file when I press the save button on excel.

The code I have is below.

Regrads

Sub WriteFixed()

Const MyPath = "C:\documents and settings\Mark\desktop\"
Const WriteFileName = "Trial1.txt"

Const ForReading = 1, ForWriting = 2, ForAppending = 3

Const TristateUseDefault = -2, TristateTrue = -1, TristateFalse = 0

Set fswrite = CreateObject("Scripting.FileSystemObject")

'open files
WritePathName = MyPath + WriteFileName
fswrite.CreateTextFile WritePathName
Set fwrite = fswrite.GetFile(WritePathName)
Set tswrite = fwrite.OpenAsTextStream(ForWriting, TristateUseDefault)

LastRow = Cells(Rows.Count, "A").End(xlUp).Row

For RowCount = 1 To LastRow
LastCol = Cells(RowCount, Columns.Count).End(xlToLeft).Column
OutPutLine = ""
For Colcount = 1 To LastCol
Data = Trim(Cells(RowCount, Colcount).Text)
If Len(Data) < 12 Then
Data = Data & WorksheetFunction.Rept(" ", 12 - Len(Data))
Else
Data = Left(Data, 12)
End If
If OutPutLine <> "" Then
OutPutLine = OutPutLine & " "
End If
OutPutLine = OutPutLine & Data
Next Colcount
tswrite.writeline OutPutLine
Next RowCount

tswrite.Close

End Sub