Save As button with name generated from field names

Hi,
 
I've created a pdf form in livecycle and added in a "Save As" button using the code: app.execMenuItem("save As")
 
I would now like to take this to the next stage of enabling the form to save based on the responses inputted in the form.
 
I would like the form to save with the following file name:
 
"PCR-Firstname + Surname"
 
First name and surname will be entered into the form by the user. PCR should be fixed and not changed.
 
I'm a novice with Javascript so your guidance is very much appreciated.
 
Thanks,
Anthony