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):
It would be nice if fetch tool will send some fixed User-Agent header, or event let user specify it via parameter.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.