You have multiple domains for your landing page CNAMES and want to choose which one is used for a landing page.
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.
Who This Solution Applies To
Customers using multiple landing page domains