JSFiddle - React, Tailwind, and code Playground

HTML

<div class="formarea">
    <select id="dropdown1">
    <option value="One">"One"</option>
    <option value="Two">"Two"</option>
    <option value="Three">or just write your own</option>
</select>
<form><input id="txtbox" type="text"></input></form>
    <input id="inputbutton" type="submit" value="Submit"></input>
</div>

CSS

.formarea{
    height:300px;
    width:300px;
    margin:0 auto;
    text-align:center;
    padding:40px;
}
#txtbox{
    margin-top:40px;
    margin-bottom:40px;
    width:200px;
}

JavaScript

var txt = document.getElementById('txtbox');
var btn = document.getElementById('inputbutton');
txt.disabled=true;

$(document).ready(function() {
    $('#dropdown1').change(function() {
        if($(this).val() == "Three"){
            document.getElementById('txtbox').disabled=false;
        }
        else{
            document.getElementById('txtbox').disabled=true;
        }
    });
});

btn.onclick = function () { 
    if((txt).disabled){
        alert('input is: ' + $('#dropdown1').val());
    }
    else{
        alert('input is: ' + $(txt).val());
    }
};