Thanks Owen,
Although a post from three years ago, I now found this topic. I have been struggling with how to print a certificate and this is one of the straightforward methods.
I have done it a bit differently: On the certificate slide I insert a hotspot. The trigger is to click on the hotspot to run the javascript. On the previous (congratulations) page, I include the instruction how to print the certificate.
An other method I used is: make a screenshot of the certificate slide, preferably with a snipping tool, paste into PowerPoint and print (or save as PDF). Advantage is that it's a full page certificate, without the title, page numer and footer text.