A URL is subdomain.domainname.com/subdirectory, where subdomain can be www. or promo. for example. Basically everything before domainname.com is a subdomain. You can have as may subdomains as you need as they act independently one from the other.
A subdirectory, also known as subfolder, is like a subpage of a particular subdomain. In order to be able to have something after the "/" when choosing the custom domain publishing method, both elements need to point to the same server, otherwise it will create a conflict and the page will not be shown.
This means that if you already have content on www.domainname.com, you can't publish an Instapage landing page on a subdirectory, for example www.domainname.com/promotion using the custom domain method, because you need to connect it to our servers and your website already points to a different server.
All the landing pages are hosted on our servers and there's no possible way to move them over to your hosting. Therefore, you will always need to create a new subdomain pointing to secure.pageserve.co in order to publish your landing page.
If you wish to publish your page on a subdirectory of your main website, for example www.domainname.com/promotion, you can use our WordPress plugin if your website is built with WordPress.
You can check out our Advanced Publishing section in order to learn more about adding a subdomain to your website.