I’m working a lot lately in Firebase, and when trying to setup Firebase Hosting for one of my domains - I wrote about other issues in an earlier post. In this case, I want to setup a subdomain (for example: subname.mysite.com), Firebase showed me the following dialog:

Firebase Hosting Dialog

In Google Domains, I tried the trick I learned in that linked article, but it didn’t work because the A record doesn’t need the whole fully qualified domain name (FQDN), all it needs is the subdomain as shown in the following figure. I whacked the domain portion from the A record and it worked like a champ.

Google Domains Custom Resource Second