JSFiddle - React, Tailwind, and code Playground

HTML

<div><input type="text" placeholder="existing"/></div>
<button class="add">add</button>

JavaScript

var add = document.querySelector(".add");
var div = document.querySelector("div");

add.addEventListener('click', function () {
    var newInput = document.createElement('input');
    newInput.outerHTML = '<input type="text" placeholder="new"/>';
    div.appendChild(newInput);
    //div.innerHTML += '<input type="text" placeholder="new"/>';

});