JSFiddle - React, Tailwind, and code Playground

HTML

Desconto
<input type="text" name="desconto" id="desconto" style="width: 100px"> Observação


<input readonly="true" type="text" name="obs" id="obs" style="width: 400px; height: 30px">

JavaScript

var desconto = document.getElementById('desconto');
var obs = document.getElementById('obs');

desconto.addEventListener('keyup', function() {
    obs.value = this.value;
});