Thanks for asking, and for working on a treat! Go to the collection profile (https://archiveofourown.org/collections/trickortreatex2021/profile) and click the "Post to Collection" button. That will take you to a new-work submission page with the collection info already added.

You'll need to add the recipient's username in the "Gift this work to" field (in the "Associations" section).

Hope that helps!