In production we use our custom url xxx.rotterdam.nl for our apps. When you type in xxx.mendixcloud.com it still works, which we don't want. Is there a way to "block" the xxx.mendixcloud.com variation?
I don't know if this is possible but the other option would be to contact Mendix and ask them to add a CNAME record and map the subdomain to the new custom URL domain. This can only be done by the owner of the top domain (DNS owner), so this should be posted at mendix.
It's not possible to configure this in the cloud portal. What you can do is insert a little redirect script in your index.html and detect if the Mendix cloud URL is used and redirect it to xxx.rotterdam.nl