To publish a page to your custom domain name or subdomain, your domain first needs to be linked to your Wishloop account. Please see the following link to: learn more about linking your domain name to your account.

Once your domain name is linked, from the campaign list screen, click on a particular page campaign and click on the "settings" tab.

In the general settings you'll be able to choose your linked domain from the dropdown menu.

PS: You can repeat the same process to un-publish a page from a custom domain name and revert it back to use the default subdomain (or publish to another custom domain).

Further Reading:

Page publishing options

How to publish a page to my custom domain name?