JSFiddle - React, Tailwind, and code Playground

HTML

<form id='f'>
    <input id='txt' type="text" placeholder='type numbers here'>
    <button http://jsfiddle.net/#tidyid='btn'>Submit</button>
</form>

JavaScript

var form= document.getElementById('f'),
    txt= document.getElementById('txt');

form.onsubmit= doStuff;

function doStuff(e) {
    if (!isNaN(txt.value)) {
        txt.value='Thanks for Interest!';
        txt.setAttribute('readonly','readonly');
    }
    return false;
}