JSFiddle - React, Tailwind, and code Playground

HTML

<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">
<input type="submit" value="Continue" name="dispatch[checkout.update_steps]">
<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">

JavaScript

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

});