I suggest you to use the specific function to achieve this, i.e. "Go To Page". It lets you select the target page to go to. Using javascript would be very tricky because you should have to manual insert the correct url that depends on the structure of your project. Anyway if you really need to use javascript consider correct url is '../<pagenum>/page.xhtml' where pagenum is the number of page to go to, but I highly recommend you to use the prebuild function "Go To Page"

Interesting point, I think you cannot use the builtin GoToPage, maybe in the future you give us an idea :)

The most simple solution coming up in mind is to create a global javascript variable called for example myEpubTotalPages and put it to project html header in this way:

window['myEpubTotalPages'] = 6

where 6 is the number of pages.

then use it in the run javascript action, such as:

var paths = window.location.pathname.split('/');
var currPageNum = parseInt(paths[paths.length-2]);
var newPage = Math.floor(Math.random() * (window['myEpubTotalPages'])) + 1;
while (newPage == currPageNum) newPage = Math.floor(Math.random() * (window['myEpubTotalPages'])) + 1;
window.location.href = '../'+newPage+'/page.xhtml';

idea here is to jump to a page between 1 and "myEpubTotalPages" avoiding current page