How to make instant launch js from chrome extension?

Question

Google Chrome

For example:
An extension that will replace the content of placeholder.
If you do this in background.js,

function first() {
  var change = document.querySelector('input');
  change.placeholder = 'Вася, пиши тут';
};
first();

the browser will almost instantly load the script. BUT, if I have slow internet or something goes wrong and background.js is loaded before this placeholder is loaded, then at least nothing will happen. And if the code is complex, something will happen, but obviously not what was expected)).
If so, do it like this

function first() {
  var change = document.querySelector('input');
  change.placeholder = 'Вася, пиши тут';
};
window.addEventListener('load', first, false);

then background.js will be loaded only after the page is fully loaded.
Question: how to load the script immediately after the placeholder is loaded?