Simple JavaScript (DOM) code not working

Question

I just finished the Javascript course and I tried on this particular code, but it doesn't work. Is something wrong? Do I need to install something first?

CODE:
https://code.sololearn.com/W3E97hAKy1fP/?ref=app

Answer 1

Or

window.addEventListener('load', function () {

let username = prompt('ENTER USERNAME');

document.querySelector('.hello').innerHTML = username;

});

Answer 2

onload = init;

function init() {
let username = prompt('ENTER USERNAME');
document.getElementsByClassName('hello')[0].innerHTML = username;
}

Answer 3

https://www.sololearn.com/post/90825/?ref=app

https://www.sololearn.com/post/94559/?ref=app

https://code.sololearn.com/WKKkpq0efxai/?ref=app