Adding a subdomain on Google Domains

Question

I'm trying to create a TXT DNS record with name _acme-challenge.www on Google Domains to allow me to validate my DNS (i.e. via _acme-challenge.www.example.com). I've already validated the domain itself; but want to validate the www subdomain so I can provide this in the certificate's Subject Alternative Name. When attempting this, the page gives error: Failed to save changes to domain settings for example.com.

Is what I'm attempting valid / if so can anyone suggest what I may be missing?

Solution

This does work:

• Navigate to Google Domains (https://domains.google.com)
• Select the domain you wish to manage
• Click the DNS icon
• Scroll down to Custom resource records
• Fill out the fields above the column heads / by the Add button (below is example data; amend per your needs)
  • Name: _acme-challenge.www
  • Type: TXT
  • TTL: 1h
  • Data: ABCDEF blah blah blah
• Click Add

I'm not sure why this didn't work the first time I tried it as I think I did exactly the above; so putting this down to a glitch. Recording here in case it's of use to others searching for similar.