Currently, alerts cannot be set up directly for bookmarked companies.
To set up alerts of interesting companies,
You can use the Track Peer Companies tracking board to stay updated on the latest activities of interesting companies. Check the article "How can I configure the Track Peer Companies board?" to configure alerts for companies.
Alternatively, you can set up alerts for the saved searches. Check the article "How to set up alerts for companies search?" to set up alerts for the saved searches.