Custom Domain with Cloudflare zero trust

Hi, I'm trying to setup Cloudflare Pages with a custom domain. I want to restrict the site to certain people, so i configured a policy which ALLOWS access to people with a certain email adress. On the overview tab of zero trust I set up the application domain's which end with *.pages.dev. However, I am unable to select a custom domain (abc.xyz.com) which is configured via CNAME: "Zone is not associated with the current account" The access page shows up correctly - but with the custom domain I can access the site directly.