After going through the first 3 steps indicated in this article in order to add your new subdomain, below are the following steps you need to take in the domain registrar before going back to finish the process and having your new subdomain in Instapage.
Click Add for a new entry, or click Modify and skip to step six for an existing entry;
Enter the Host name. For example, if you picked lander.mydomain.comas your address, enter lander here. Or, if you're using a root domain such as www.mydomain.com then you'll want to enter wwwhere;
Select DNS alias CNAME at the host type;
Enter secure.pageserve.co as the TargetHost and click Modify.
