You can't, you have to expressly state the changed port at the client end.
DNS is purely for IP resolution, it doesn't deal with port numbers at all, that's a client-side assumption that's built into your various client applications.
So for instance you'd use your browser to point to example.com:8090 - there's no other way for the client to know that you've changed the port.
By the way, please read our FAQ, this site's for professional sysadmins, be aware of our sister site superuser.com which is perhaps more appropriate for very basic general questions such as this.