Swift WKUserContentController getting callback from Javascript

I'm trying to set my app up to get a callback from a JS function. I'm seeing a number of different responses and configurations out there, and I think I'm close, but something is off.

This is the error I'm getting from the JS:

TypeError: undefined is not an object (evaluating 'window.JSInterface.writeCarKey')

And this is the JS Code:

function writeToNative(id, description, key) { 
     window.JSInterface.writeCarKey(id, description, key); }

I've tried variations of this in my app:

let contentController = WKUserContentController()
contentController.add(self, name: "writeCarKey()")

all to no avail.

The above code is working in an Android app, but from the various responses to this kind of issue I'm seeing, it looks like I need this in the JS code?

window.myApp.writeCarKey = function() {
window.webkit.messageHandlers.writeCarKey.postMessage("1", "K", "CAR"); };

Is it just this that I'm missing in the JS code?