index.html

<script>
  function inject() {
    var query = new URL(window.location);
    var code = query.searchParams.get('code');
    if (!code) {
      return;
    }

    var input = document.querySelector('input[placeholder="_ _ _ _ _ _ _ _ _ _ _ _ _ _ _"]');
    if (!input) {
      return;
    }

    var inputSetter = Object.getOwnPropertyDescriptor(window.HTMLInputElement.prototype, "value").set;
    inputSetter.call(input, code);
  }

  inject();
</script>

inject-code.js

function inject() {
  var query = new URL(window.location);
  var code = query.searchParams.get('code');
  if (!code) {
    return;
  }
  
  var input = document.querySelector('input[placeholder="_ _ _ _ _ _ _ _ _ _ _ _ _ _ _"]');
  if (!input) {
    return;
  }
  
  var nativeInputValueSetter = Object.getOwnPropertyDescriptor(window.HTMLInputElement.prototype, "value").set;
  nativeInputValueSetter.call(input, code);
}

inject();