Creating filename from field data and saving

I have a field on my form in which I use javascript to generate a reference number based on the date and time when the document opens. I would like to use this number as the filename with a "SaveAs" button. Ideally, the end user would open the form, the number is generated, the end user fills out the rest of the form and clicks the "SaveAs" button. The save dialog box would open with the reference number already in the filename field, rather than the original filename. The end user could then direct the file to be saved to his preferred location. I have tried working with doc.SaveAs and trusted functions, but have not been able to accomplish this. Is it possible?