How can I setup custom login URL for Zoho mail on google cloud?

There is a brief "how to info" for custom login URL out there, which is;

1.Add CNAME in the DNS Manager for the subdomain (Ex: mail.yourdomain.com)
2.Point it to business.zoho.com

Name/ Host/ Alias/ CNAME	Value/ Points To/ Destination/CNAME
mail

I can't figure out how to on Google Cloud. Adding a CNAME record window is like this;

Help will be appreciated..