Bookmarking is not available now in Ghost. The best place to submit a feature request would be over in the Ideas category. Maybe another mod could move this thread over there.
If you are interested in this feature, I recommend voting for the ActivityPub idea.
Bookmarking is one feature of ActivityPub apps.
Also, a bookmarking feature Ghost would only work for one blog at a time, but if Ghost were federated over ActivityPub, you could use a single account to bookmark every Ghost blog, Mastodon posts, Peertube videos, Pixelfed photos, Lemmy posts and some WordPress posts and more… all from one account!