First, you will need to make sure your secondary domains are set up with a CNAME and a domain alias.
You don't choose a domain when creating the page. All pages are created under the default/original landing page domain, but they're all available from all domain aliases as well. Swapping out the default domain with the domain alias in the URL will take you to the same page. You find more detail on this here - Multiple Domains (CNAMEs) in a Single Marketo Instance