You can populate a variable using a query string parameter.
In most cases you use a plug-in to populate variables from the query string. If a typo or similar issue prevents the value from being populated, you can populate the variable using processing rules.
You should always check to see if a value is empty or contains the expected value before you overwrite it.
|Condition||Campaign is Not Set|
|Action||Overwrite value of Campaign to Query String Parameter cpid|