
ps.
Not sure if a picklist is going to be an option.....
6 answers
I would think about using Campaigns, or maybe create a Custom "PromoCode" object and maybe lock down who can create new Promo Code records to Admin and/or Marketing. Then create a (required) custom field on the Lead object and select
Datatype = Lookup
Object = PromoCode
Another option would be a Validation Rule that uses a REGEX funtion but they can be kinda tricky