JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-1.10.2.js"></script>
<table>
<tr>
            <td>Payment amount:</td>
            <td style="text-align: right; vertical-align: middle;"><input name="amount" id="amount" style="width: 150px; height: 22px;" type="text" /> <br />
            </td>
            <td style="text-align: right; vertical-align: middle;">&nbsp;<span style="color: #ff0000;">*</span></td>
        </tr>
        <tr>

        <tr>
            <td>Payment total including 2.5%:</td>
            <td style="text-align: right; vertical-align: middle;"><input name="pay_amount" id="pay_amount" style="width: 150px; height: 22px;" type="text" disabled /> <br />
            </td>
            <td style="text-align: right; vertical-align: middle;">&nbsp;<span style="color: #ff0000;">*</span></td>
        </tr>
</table>

JavaScript

$(document).ready(function(){
    $('#amount').keyup(function(){
        $('#pay_amount').val($('#amount').val() * 1.025);
    });   
});