If you see a post you want to share but you want to add a Content Warning (CW), it is now very easy to do this:
1. Go to the post you want to share
2. Quote the post (on the web interface click 🔁 Boost and then "Quote")
3. Add a CW to your own post, the one that appears above the post being quoted. This will hide everything in your post including the quoted post.
This used to be impossible, but since Mastodon added quoting it also allows adding CWs to posts you want to share.