JSFiddle - React, Tailwind, and code Playground

HTML

<h3>Enter Text:</h3>
<form id="myForm" method="post" onsubmit="onsubmit_handler()">
    <input type="text" id="data" name="data" value=""/>
    <input type="submit" value="Post"/>
</form>

JavaScript

function onsubmit_handler(){
    var myForm = document.getElementById('myForm');
    var data = document.getElementById('data').value;
    if(data == "a")
        myForm.setAttribute('action', 'a.php');
    else if(data == "b")
        myForm.setAttribute('action', 'b.php');
    else
        myForm.setAttribute('action', '2.php');
}