JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <input type="text" id="name" />
    <input type="text" id="url" />
</form>

JavaScript

addEvent(document.getElementById('name'), 'keyup', function () {
   document.getElementById('url').value = this.value.replace(' ', '_');
});


function addEvent(ele, evnt, funct) {
  if (ele.addEventListener) // W3C
    return ele.addEventListener(evnt,funct,false);
  else if (ele.attachEvent)  // IE
    return ele.attachEvent("on"+evnt,funct);
}