Extension for Chrome - How to use the DOM on the site that is open

Question

How do I use the DOM of a page that is open in the browser by the code in my extension?

Ex: In the popup of my extension has a button, which I hope when clicking, the value of a div on the open site changes.

Can you do this? I can only access the DOM of the popup extension, but I can’t access the DOM of the open page

Answer 1

I have an extension for Chrome and I did something like this:

document.addEventListener('DOMContentLoaded', function () {
  var btnLoginCI = createButton('Login CI');

  btnLoginCI.addEventListener('click', function () {
    alert('clicked'); // code here!!!
  });
});