Send Custom WooCommerce Billing Field to PickList

I have a custom County woocommerce checkout field being added to billing.

add_filter( 'woocommerce_checkout_fields' , 'custom_billing_fields' );
function custom_billing_fields( $fields ) {
	$fields['billing']['billing_county'] = [
		'label' => 'County',
		'type' => 'select',
		'options' => [
			'Adams' => __('Adams'),
                        ......
                 ]
         ]
}

But cannot find a way to access this data to send to a PickList in SalesForce. Is there anything special I have to do for nested fields like ['billing']['billing_county']?

Thanks!