Answer 1

<textarea id="text"></textarea>
<script>
var str = prompt("Enter a sentence");
document.querySelector("#text").value = str;
</script>

https://code.sololearn.com/W11TDJxY6fx9/?ref=app

Answer 2

Enclosed your initial run code in
onload = function() {
// start up code
}

https://code.sololearn.com/W0esomd53ITw/?ref=app