Flagged by Jose Jimenez:
These properties are exactly the same as the ones you can specify in your portal-ext.properties. Make sure to visit the Content Delivery Network section of the portal.properties reference document if you don't know how to fill in the CDN fields. Once you're finished, click Save and your old host is replaced with your new CDN host for static content.
Comment: "We should warn that the value for CDN Host must always include the full protocol (http://cdnexample1.liferay.com or https://cdnexample.liferay.com) This information is in the properties description inside the portal.properties, however configuring via UI you do not have an error message or similar and the customer can goes into issues (we have some tickets due this problem). We should update this documentation for all the Liferay versions 7.x"