Doug Robbins
posted a macro a long time ago that gives us the basic code which can be adapted to your requirements. I've added an arbitrary limit so your default filename doesn't get too long.
Code:
Sub FeedMeSeymour()
Dim fd As FileDialog
Set fd = Application.FileDialog(msoFileDialogSaveAs)
With fd
.InitialFileName = Left(Selection.Text, 50)
If .Show <> 0 Then 'if the user didn't press the cancel button
.Execute
End If
End With
Set fd = Nothing
End Sub