You shouldn't need to create tracking codes in Adobe, rather those codes should be following a standardized format.
So think of UTMs for a second, you have UTM Source - which serves a specific need of the source, or UTM Campaign - which is the unique campaign code so you can tell your various campaigns apart.
CID is very similar, except that all of the items are passed in a single string, in a specific order, with a specific delimiter. This format (or taxonomy) is the key to how this will work.
If this is coming from a previous vendor, there is likely already rules and formatting concepts in place.
This is an example (but not necessarily what is being used):
facebook:social:october-2022-sale
or
facebook_social_october-2022-sale
The delimiter can be : or _ or anything that you want... and the order of the items in this case is source:medium:campaign - but the important thing is that the use is consistent and that the order is set up properly - since you will be using Regex to parse this into it's individual parts.
Here is a good article: https://analyticsdemystified.com/adobe-analytics/using-utm-campaign-parameters-adobe-analytics/
This is why the first thing I asked about was if you had a taxonomy - since you are taking over an existing implementation... most of the processing rules should already be in place, for an existing defined CID format.
I don't know what the "key" is that you mentioned....