This is a Lightstream Labs feature. For info on enabling Labs features please visit this article
To add your Streamlabs (TwitchAlerts) asset, visit the Streamlabs website to first copy the url of the widget you would like to add.
Once you have this copied, return to your Lightstream project, click the “Add Layer” button, and then click on "Alerts". Afterwards, you'll want to select Streamlabs and then paste the URL you copied earlier in for your layer.