I'd like this too - did you find a way?

Here is what I've found out. You can set the url and it is remembered - i.e. you can extract the correct url sometime later. However it is not the url the Reminders app lets you configure. Also if you set a url in the Reminders app it does not get set into the EKReminder.url. I can only conclude that EKReminder has some unpublished url property which is unconnected to the base class url. You could 'go fishing' for this unpublished interface there is little point as it would be not allowed by the App Store review process.

Did you figure this one out? I've just been trying to set the URL programatically but without success.