Maybe - and that's a wild guess - the JavaScript code is running before the HTML is loaded, and it can't find the input field. Check if the script tag is before the form and change it to be after. Or add an event listener to run after the page is loaded:

document.addEventListener("DOMContentLoaded", function(event) { 
  // your code here
});

But as I said, I'm just guessing, it'd be nice to see the HTML to confirm that

hkotsubo‭

I already tried that before asking the question from a user script manager (I also tried listening to the load event) but it didn't help.

Even if I put the value directly after everything was loaded via the browser console, the problem persists so something prevents the value from being recognized, if not typed manually.

deleted user Could you add a link to the website, so we can at least check the HTML and try to see what the problem is? Because your code should work. Just double checking the basic stuff: what does document.querySelector("#example") return? When you run your code, does any error message appear in the console? etc. With the information provided so far, it's hard to guess, or even reproduce the problem...