public function copyValueFromOtherStep(FieldList $fields, $formStep, $fieldName, $fieldNameTarget = null)
    {
        // if a target field isn't defined use the same fieldname
        if (!$fieldNameTarget) {
            $fieldNameTarget = $fieldName;
        }

        $fields->fieldByName($fieldNameTarget)->setValue($this->getValueFromOtherStep($formStep, $fieldName));
    }