I published my game, and it launches fine whenever I hit the 'Play' button from my creation tab. But, if I attempt to launch the game using the 'Play Online' button, it gets stuck loading forever.

I understand that it is missing a .js file, but I do not know how to make or attach that to my game.

We had this problem when version 5.8 was uploaded, but it got fixed (around 26/Aug). It seems to have reappeared. We will take a look.