def _add_extension_feed_item(client, customer_id, callout_text, language_code):
    """Creates a new extension feed item for the callout extension.

    Args:
        client: an initialized GoogleAdsClient instance.
        customer_id: a client customer ID.
        callout_text: a str of text for a hotel callout feed item.
        language_code: the language of the hotel callout feed item text.

    Returns:
        a str resource name of the newly created extension feed item.
    """
    extension_feed_item_operation = client.get_type(
        "ExtensionFeedItemOperation"
    )
    extension_feed_item = extension_feed_item_operation.create
    extension_feed_item.hotel_callout_feed_item.text = callout_text
    extension_feed_item.hotel_callout_feed_item.language_code = language_code

    extension_feed_item_service = client.get_service("ExtensionFeedItemService")
    response = extension_feed_item_service.mutate_extension_feed_items(
        customer_id=customer_id, operations=[extension_feed_item_operation]
    )
    resource_name = response.results[0].resource_name
    print(
        f"Created an extension feed item with resource name: "
        "'{resource_name}'"
    )
    return resource_name