First, you have to register a Service Worker on your page.

​

// index.html

window.addEventListener('load', function () {

    navigator

        .serviceWorker

        .register('/service-worker.js');

});

//In the Service Worker a request is captured, modified and forwarded by calling the WindowOrWorkerGlobalScope.fetch() method.

​

// service-worker.js

self.addEventListener('fetch', function (event) {

    event.respondWith(async function () {

        let headers = new Headers()

        headers.append("X-Custom-Header", "Random value")

        return fetch(event.request, {headers: headers})

    }());

});