JSFiddle - React, Tailwind, and code Playground

HTML

<a id="reflectedlink" href="http://www.google.com">http://www.google.com/</a>
<input id="username"/>

JavaScript

var link= document.getElementById('reflectedlink');
var input= document.getElementById('username');
input.onchange=input.onkeyup= function() {
    link.pathname= "/"+input.value;
    link.firstChild.data= link.href;
};