To add your Streamlabs (formerly TwitchAlerts) layer, visit the Streamlabs Widgets Dashboard area 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 "3rd Party Integrations". Afterwards, you'll want to select Streamlabs and then paste the URL you copied earlier in for your layer.
Note: If the widget you're using has test buttons (like the Alertbox for example), you can use them to test if it's working properly in your Lightstream project space.