I'm assuming that "bookmarking" a post is not a problem for you. The underlying question is how to structure so that you can show the bookmarked activities for a user?

Regarding how to "bookmark" a post, same as the recommendation from @Robot-43, you can add it as a reaction to the post.

Regarding how to structure so that you can have a page that lists all the bookmarked posts, here is my suggestion:

You can get all reactions of a userId and filter it by kind https://getstream.io/activity-feeds/docs/dotnet-csharp/reactions_introduction/#retrieving-reactions

You can read reactions and filter them based on their user_id or activity_id values. Further filtering can be done with the kind parameter (e.g. retrieve all likes by one user, retrieve all comments for one activity, etc.).

Request an API to get all the reactions filtering by the user's ID, and the "bookmark" reaction kind, now you get a list of activityIds with that reaction.

Next make a request to the batch activity get API, and send the list of activityIds in the request. https://getstream.io/activity-feeds/docs/dotnet-csharp/add_many_activities/#batch-get-activities-by-id

You can also find the batch enriched activity get in the source code probably, even if it's not documented in the website (if enriched activities is what you are using)

And then, you'll have your page :)

in batch activities get, you can send 100 activityIds at a time.