JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="event_content">www.london2012.com</textarea>

CSS

textarea {
    width:350px;
    height:100px;
}

JavaScript

$(function(){
    var old = $('#event_content').val();
    
    var news = '<a href="http://'+old+'" target="_blank">'+old+'</a>';
    
    $('#event_content').val(news);
});