All URLs consist of three components: a subdomain, a domain and sometimes a subdirectory. For example, if the URL is hello.website.com/page, then "hello" is the subdomain, "website.com" is the domain, "page" is the subdirectory.
In order to publish on Instapage, you need to create a subdomain set up in the form of a CNAME type record that points to our server address of secure.pageserve.co, as instructed in the following article: Publishing on a Custom Domain (CNAME)
You can do the above and name your subdomain anything that you wish, such as "promo", "campaign", "hello" or "www". Even though most websites are published on "www", it is simply another subdomain and you can use it for publishing page experiences built with Instapage if you configure it accordingly. Note that you can only publish on "www" if you do not already have a website or another webpage on it. To publish on the "www" subdomain of your main domain first follow the instructions in this article. Afterwards, you will need to configure it in your DNS settings as per the article mentioned in the previous paragraph.
Because some domain registrars create a default record for your "www" subdomain that points to the root domain, you will need to check for that first, as you cannot have two records for the same subdomain. The example below is this situation shown in GoDaddy:
1. You can see the default "www" subdomain created
2. You can click on the Edit symbol on the right side to either delete the record or edit it to make sure it points to secure.pageserve.co.
Some registrars might create the default record as an A record or another type, not necessarily a CNAME. If you cannot edit that, delete the existing record for "www" and create a new one. Make sure the new one is of the type "CNAME".