Save As... new filename derived from field

I've got a Save As button in my form which works fine with the following script:

app.execMenuItem("SaveAs")

But because it's just performing the standard 'save as' function, it displays the 'save as' window with the 'filename' automatically populated with the .pdf form name. Therefore, if a user just clicks the Save button, it will override the original file.

Can you please tell me how I can either have it display an empty 'filename' field, or even better, populate it with text from a selected field within my document?

Thanks, Peta