I’ve searched forum, but could not find duplicate to upvote.
Google Domains support dynamic dns updates, but it requires User-Agent header field.
https://support.google.com/domains/answer/6147083?hl=en says (in folded section):
Note: You must set a user agent in your request as well. Web browsers will generally add this for you when testing via the above url.
It would be nice if fetch tool will send some fixed User-Agent header, or event let user specify it via parameter.