Insert a value into Database before checkout

[babyphp]

Hi, 

I am looking for a solution so that customers can choose VAT before placing an order.

 

I have created one more column getvat in table tblorder.

Can you help me how to write the code? I am a newbie WHMCS.

Thanks so much.

<?php
function getvatcheckout($vars)
{

.....

}
add_hook('ClientAreaPageCart', 1, 'getvatcheckout');

?>