JSFiddle - React, Tailwind, and code Playground

HTML

<div id="invoice">
<form onSubmit="home.php" style="padding:0;margin:0;" method="post"> 

<select name="location[]">
<option selected="selected" style=" display: none;">Location of item(s)...</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
<input type="button" style="position:static;left:115px;" onclick="add_fields();" value="+" />

<script>
function add_fields() {
    document.getElementById('invoice').innerHTML += '<select name="location[]"><option selected="selected" style=" display: none;">Location of item(s)...</option><option value="1">1</option><option value="2">2</option><option value="3">3</option><option value="4">4</option><option value="5">5</option><br>';
}
</script>

JavaScript

<?php
if ($_SERVER['REQUEST_METHOD']==='POST') {
    $locationArray=$_POST['location'];
    
}
?>