Creating a filename for a form after executing 'save as' script

I have a feeling this has been asked before, but the closest I've found to something similar was this:
http://www.acrobatusers.com/forums/aucbb/viewtopic.php?id=22934

But I didn't understand Radzmar's response. How do you create a script at folder level?

I'll clarify what I'm after.

When the user has completed the form, they click a 'save as' button to enable them to save their file.
I've got that working fine with the following:

form1.ExtraTraining.SaveFormAsBtn::mouseUp - (JavaScript, client)
xfa.host.messageBox("Please save this form with your own file name","Reminder",1);
app.execMenuItem("SaveAs");

but then under that I've got...
// SaveAs and close this document
mySaveDoc("TACPg1.TACSF.LastName.pdf");

...this is to try to pick up the user's name from the 'LastName' field and place it into the Filename: field when the 'Save As' dialog is displayed. Only it doesn't work.

What I'm trying to achieve is that when the user clicks the 'save as' button, it not only prompts them to do so, but loads something other than the original template name into the filename field - so that the template can't be overwritten. But also so that when they submit the form, it will be clear by the filename who it's from.

Any thoughts?