Create a Save As button in a form that calls on a field for its file name

"You can't pre-populate the File Name field in the Save As dialog" - Not directly but there is a workaround when you combine app.browseForDoc (which reads 'Save As' in the popup window, with the file name prepopulated) with doc.SaveAs.  This requires a priviliged context as well.  Example:

try{var pth=app.browseForDoc(true,this.getField("File Name").value+".pdf").cPath}catch(e){var pth=null};
if(pth)
{this.saveAs(pth)}