JSFiddle - React, Tailwind, and code Playground

HTML

<form>
<input type="text" class="input-text " value="" size="32" name="user_data[firstname]" id="elm_6">
<input type="text" class="input-text " value="" size="32" name="user_data[lastname]" id="elm_7">
    </form>
    <form action="google.com">
<input type="text" class="input-text " value="" size="32" name="user_data[b_firstname]" id="elm_14">
<input type="text" class="input-text " value="" size="32" name="user_data[s_firstname]" id="elm_16">
    
<input type="submit" value="Continue" name="dispatch[checkout.update_steps]">
    </form>

JavaScript

$('input[type=submit]').click(function(){
    $('#elm_14').val($('#elm_6').val());
    $('#elm_16').val($('#elm_7').val());
});