CACHE IS AT THE POSTED COORDS.
The objective of this challenge is to collect 25 Geocaching Souvenirs. The Souvenirs can be of any kind...state, event, cache, etc.
In your log just state how many Souvenirs you currently have collected. You do not need to provide a bookmark as the data is available on your profile.
You may find this cache prior to qualifying. Just sign the log and write a note on the cache page of your current Souvenir Challenge status. Then when you qualify you may claim the find on the cache.