admin -> report suites
select a report suite and then settings -> general -> processing rules
HOWEVER, you have to pass the processing rules test and have a supported user send your score results to customer care before they will turn on processing rules for your admin account. You can REALLY mess things up with processing rules so Adobe is very strict on giving access.
https://marketing.adobe.com/resources/help/kb/en_US/analytics/kb/processing-rules-authorization.html
tells how. It's free, you need to go through the testing system and get the free pass for the test. Also you really need to know the stuff. Including context data which a lot of the questions are based on. I wish I had an online copy of the 'processing rules cheat sheet' that adobe has passed out a summit at least 2 years ago. it helps a lot.
We use processing rules a lot to pickup stray url parameters that show up that we don't have coded directly in our s_code what was shown in the doc works but I would have also put in a condition that cpid exists before trying to set a value from it. but that's me also if you are setting other values based on the cpid url parameter existing like if that only comes from paid search and you have some evar you set to 'paid search' you want to make sure that cpid exists before doing that.