Real Beginner Question

I have been using Chronoforms 5 and have upgraded to 6 because I need my registration form to auto activate which no-longer works with 5.
The problem is things have changed and what I knew before I no-longer seem to know.

I have three fields firstname - lastname - fullname
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On submit I used the following code in a "Custom Code" action[pre]<?php
$form->data['fullname'] = "{$form->data['firstname']} {$form->data['lastname']}";
?>[/pre]
I can't seem to get this to work anymore.
I've also tried it as custom code again and with the PHP action. I've also tried it like this:

$form->data['fullname'] = "{$form->data['firstname']} {$form->data['lastname']}"

as it says to not use the tags.

Could someone please tell me how to do this the v6 way?
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Thanks in advance.[br][br]Oh, and my Checkbox seems to be a mutation of the Chronoforms one and my templates one and doesn't work???