Main/resources/[phone]/roadphone/public/custom_apps/app.js

class CustomApp {
    constructor() {
        window.addEventListener('message', (event) => {
            if (!event.data) return
            const e = event.data
            if (e.customevent === 'test') {
                console.log(`TEST!`)
            }
        })
    }
}

const customApp = new CustomApp()