Hi,
To do that you must use some custom js. If you use the latest version of DFFS you can trigger a rule “in reverse” from custom js, if not you must create a new rule that has the trigger set to “No trigger, must be triggered manually” and then call this rule like this form custom js:
var myUrlKey = GetUrlKeyValue("TheUrlKey");
if(myUrlKey === ""){
// Not found - trigger the rule "in reverse" if you have the latest version of DFFS
spjs.dffs.triggerRule(["~IndexOrFriendlyNameOfRule"]);
// Or trigger another rule
spjs.dffs.triggerRule(["IndexOrFriendlyNameOfTheReversedRule"]);
}