Running a script which reads DOM elements and passing them back to worker. MV3

Question

I'm changing from MV2 to MV3. Thought it would be easy, but for some reason my web scraping script isnt being triggered.

I have only 2 scripts, popup.js and payload.js

payload.js parses elements and gets values.

popup.js does the actual processing.

Previously, in MV2 the payload script was executed on load by:

window.addEventListener('load', (evt) => {
    chrome.extension.getBackgroundPage().chrome.tabs.executeScript(null, {
        file: 'payload.js'
    });
});

However, now in MV3 this is no longer working and for the life of me i cant figure out how to make it run.

The payload.js basically sends these across to the popup.js like so: chrome.runtime.sendMessage({name: PRICE_LABEL, content: price});

The popup.js intercepts the call and does the data processing:

chrome.runtime.onMessage.addListener(async function (field, content) {
   // do stuff here
});

Thats pretty much all i'm trying to do, but i cant figure out how to.