Where to add custom field code

I found the code to add custom export field. But not sure which PHP I need to add this code to or under what session.

add_filter(‘woe_fetch_order_row’, ‘woe_modify_order_row’, 10, 2);
function woe_modify_order_row($row,$order_id) {
if($row[‘abc_field’] ne ”)
$row[‘custom_field’] = 0;
else
$row[‘custom_field’] = 1;
return $row;
}

Thank you