Function Keys Interactivity in Storyline Using JavaScript

I have been struggling with this too and need an assist.  I have created an AS400 step-by-step simulation in Storyline 2 that requires users to press the F4 or F6 key to move to the next slide.  I have executed this javascript: document.onkeydown = function (e) {if(e.which == 115){return false;}} for F4. (I changed 115 to 117 for F6)  They appear to be disabled but they don't advance to the next slide, even when I add a second trigger to jump when the user presses the key.

Any ideas?? My workaround of placing a hotspot over the function onscreen to jump fell flat with my test group because they were able to execute other activities using their keyboard in the training. 

Thanks in advance!