When going to publish your page after you are done working on it, you're presented with multiple publishing options. One of those is "Demo Page".
We offer this subdomain in order for you to be able to test your page before you publish to your own custom domain.
To give your page a testing URL, click on Demo Page, enter a subdomain in the given text field, then click on Save and Publish and a modal will appear with the URL of your page.
We do not recommend running ads or any other campaigns to a pagedemo URL. This domain should only be used for testing purposes!
To change this URL when you're ready to go live, Unpublish the page, then Switch Method and select one of the other publishing options.