I assume that when you say "user only has to select the concerned txt file
in a form", you mean that when the user has the Label type mailmerge main
document open, they only have to select the data source by using something
like the file open dialog.

If that is the case, the following will probably do what you want, though
you may have to alter the text that is being inserted into the text document
so that the field names and the separators are appropriate for your
situation:

Dim opened As Boolean
Dim maindoc As Document, source As Document
Dim sourcename As String
Set maindoc = ActiveDocument
If maindoc.MailMerge.MainDocumentType <> wdMailingLabels Then
   MsgBox "The active document is not a label type mailmerge main
document."
   Exit Sub
End If
opened = Dialogs(wdDialogFileOpen).Show
If opened = False Then
   MsgBox "You pressed cancel."
   Exit Sub
End If
Set source = ActiveDocument
sourcename = source.FullName
If UCase(Right(sourcename, 3)) <> "TXT" Then
   MsgBox "The file that you opened is not a text file."
   Exit Sub
End If
source.Range.InsertBefore "first,last,address,city,postcode" & vbCr
'modification may be required
source.Save
source.Close
With maindoc.MailMerge
   .OpenDataSource Name:=sourcename
   .Destination = wdSendToPrinter
   .Execute
End With