JSFiddle - React, Tailwind, and code Playground

HTML

Name: <input type="text" id="name">
<br/>
Area: <textarea id="area"></textarea>

JavaScript

$("#name").on("input", function(){
  var name = $(this).val();
  var msg = "Your message here";
  var content = "Dear "+name+", \n" + msg;
  $("#area").text(content);
});