To point your domain from Google Domains to your SITE123 website:
Go to Google Admin console and Sign In.
Enter your domain name then click Go.
At the bottom of the page, select More Controls >> Domains.
On the Domains page, select Add/remove domains, then select Advanced DNS settings.
On the Advanced DNS settings page, click Sign in to DNS Console. Copy the Sign-in name and Password information, you'll need it for the next step.
Log in to the Google Domain Manager using the Sign-in name and Password from the Advanced DNS settings page.
On the domain_name page, in the Host Records section, click Edit.
In the Host Records section, click Add New.
Add A record as 18.104.22.168 and a CNAME of www pointing to ssl.site123.com
Click to Save Changes.
Note: Google Domains is an external domain service and the steps of connecting your domain may change over time as Google updates its services.