Javascript/HTML Element - additional documentation - isTrusted- where to find

I just ran into a problem. I wanted to write a small script that wrote into an input field by simulating key presses. But it doesn't work.

<input type="text" id="mainInput" />

const inputElement = document.getElementById("mainInput");

for (let i = 0; i < 100; i++) {

  const newEvent = new KeyboardEvent("keypress", {
      charCode: 115,
      key: "s",
      which: 115
  });
  inputElement.dispatchEvent(newEvent);
}

I also attached an event listener and could confirm that my event is triggered correctly. I assume it is because the isTrusted flag is false and I understand that I cannot change that when I create an event via javascript.

But I skimmed now twice over the documetation of the input and text input field on MDN. In it has no mention of that. Because of that I cannot be sure if it really is the isTrusted flag or if I do something wrong. I am pretty convinced of it, but that's still different from having definite confirmation.

So, my question is two-fold: 1. Is it the isTrusted flag why it isn't working? 2. Where could I have read about it? Did I overlook it on MDN? Or is it hidden in some obscure RFC?