JSFiddle - React, Tailwind, and code Playground

HTML

<form name="myForm" method="post" action="http://google.com" id="myForm" onsubmit="submitForm()" >

    <select name="modeOfT" id="choice">
        <option value="A">A</option>
        <option value="B">B</option>
    </select> 

    <input type="submit" name="submitBtn"/>
</form >

JavaScript

function submitForm(){
    var form = document.getElementById("myForm");
    var mode = document.getElementById("choice");
    if(mode.value == "B"){
        form.action = "http://msn.com";
    }
}