Zulip supports integration with Basecamp and can notify you of events in Basecamp.

First, create the stream you'd like to use for Basecamp notifications, and subscribe all interested parties to this stream. We recommend the name basecamp.

{{ external_api_uri_subdomain }}/v1/external/basecamp?api_key=abcdefgh&stream=basecamp

where api_key is the API key of your Zulip bot, and stream is the stream name you want the notifications sent to.

Next, go to your project on Basecamp and choose Set up webhooks from the Settings menu that is located in top right corner.

Click on Add a new webhook button

Paste the URL you created above and choose which events you want to be notified about. Make sure that Enable this webhook? is checked. Click Add this webhook.

Congratulations! You're done!
When you do things in basecamp action, you'll get a notification like this: