At the top of the page, confirm that the Google Domains tab is set to "Google Domains (Active)". (if the Custom tab is set to "Custom (Active)," you use custom name servers and must configure resource records with your actual DNS provider).
Under “Resource records,” click Custom records and then Manage custom records.
Here you will add the TXT record:
Host name: _dmarc
Type: TXT
TTL: 60 (1 minute - so that you do not need to wait too long before verifying)
Data: "v=DMARC1; p=none; rua=mailto:rua-import-xxxx@sendforensics.com" [Notes: replace rua-import-xxxx@sendforensics.com with your own unique SendForensics RUA processing address which can be found in Domain Settings for your domain.]
Click Save.
Wait for a few minutes, then you can verify that you have the correct record entered in Domain Settings for your domain.