1. Log in to your register.com account;
2. Go to Manage Account;
3. Hover your cursor over Domains and select Manage;
4. Click Manage under the domain you want to add a subdomain to;
5. Scroll down and click on Edit Domain Aliases Records next to CNAME;
6. Now just give your subdomain what name you want and make sure it points to secure.pageserve.co, then click Continue.