To add your Singular.live asset, visit the Singular.live website, log in, and select the app you'd like to add as an overlay. Then you'll need to click the monitor icon in the top right and copy the url provided for your overlay.
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 Singular.live and then paste the URL you copied earlier in for your layer.