JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" value="" id="txt_url" />

JavaScript

var el=document.getElementById('txt_url');
el.onblur=function()
{
    var str=el.value;
    if(str=='') return;    
    if(str.indexOf('http://')==-1) 
    el.value='http://'+str;    
}