This is a Lightstream Labs feature. For info on enabling Labs features please visit this article
To add your Muxy alerts, visit the Muxy 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 Muxy and then paste the URL you copied earlier in for your layer.