JSFiddle - React, Tailwind, and code Playground

HTML

<html>
<head>
<script>
function FillBilling(f) {
  if(f.billingtoo.checked == true) {
    f.billingname.value = f.shippingname.value;
    f.billingcity.value = f.shippingcity.value;
  }
}
</script>
</head>
<body>
  <b>Mailing Address</b>
<br><br>
<form>
Name:
<input type="text" name="shippingname">
<br>
City:
<input type="text" name="shippingcity">
<br>
<input type="checkbox" name="billingtoo" onclick="FillBilling(this.form)">
<em>Check this box if Billing Address and Mailing Address are the same.</em>
<P>
<b>Billing Address</b>
<br><br>
Name:
<input type="text" name="billingname">
<br>
City:
<input type="text" name="billingcity">
</P>
</form>
</body>
</html>