Hi,
You would need to turn off the required setting of the custom field, and instead, implement the event onBeforeHikaUserRegistration(&$ret, $input_data, $mode) in a plugin of the group "hikashop".
If $mode is equal to 2 then you're in guest mode, and otherwise, you're in registration mode.
In
you'll have an object with the user custom fields. And in
you'll have an object with the address custom fields of the billing address. And in
$ret['shippingAddressData']
you'll have an object with the address custom fields of the shipping address.