How can I implement a repost button?

Feed APIs support repost with activity reactions and feed targets. The way this works is that you create a reaction called "repost" and specify the current user's user feed as a feed target. When you do that the user feed will get a repost activity with the reposted activity embedded in the object field.

A React example of how to implement a repost button is given here

https://github.com/GetStream/react-activity-feed/blob/main/src/components/RepostButton.tsx