There’s a line in web/mygame/index.html:
// INSERT store name; disabled by default because it's confusing for newbie authors
window.storeName = null;
You can set that storeName to something of your choice; use all lower case letters, no spaces.
window.storeName = "victorgijsbers";
From then on, the game will preserve state when you refresh the page.
I recommend leaving this off (leaving it null) during development of the game, because it can be very confusing to preserve state while you’re rewriting.
When people submit Hosted Games to us, we turn it on ourselves when we post the game to our web site.