How to extend the the page Journey object?

Elastic Observability Synthetics
1

Currently, our application is required to inject the special header, and if I put the code into synthetics.config.ts, it will show the value in the elastic portal without hiding it, which is not ideal. Therefore, we explore one of the methods, which is injecting the header into the page keyword in each of the journey files. It will be ideal, and it does not expose any value from the elastic portal.

playwrightOptions: {
       extraHTTPHeaders: {
         "id": process.env.id,
         "secret": process.env.cs,
       },
    },

In PW, we allow to extend the test method

import { test as base } from "@playwright/test";
export const test = base.extend<{ __initialize: void }>({})

However, I can't apply the same way to extend the journey.

import { journey as base } from "@elastic/synthetics";
export const test = base.extend< { __initialize: void }>({});

Q: is there any example we can extend the page keyword globally?