You can use my user script: /t/Userscript-Stroke-Order-Diagram/5040/1
If you don’t know how to install user scripts we can guide you through that (I wonder if there’s a guide here on the forums so we can just link new people to that…)
As for why WaniKani doesn’t teach it, I believe the official statement said that it’s simply because WaniKani makes no effort to teach writing, so they never bothered to implement that either. I doubt that they’re strictly against that feature, it just wasn’t on their agenda.
EDIT: Ninja’d… it’s my own script god dammit, don’t steal my thunder T_T