Technically, it is called “Live Assist for Dynamics 365 Powered by CaféX” (hands up who prefers “Web Chat”) and is now available on AppSource.
Full details of the announcement are available. As per AppSource guidelines, there is a free trial and a video walkthrough.
This app will allow you to embed web chat operator panel within web client and Unified Service Desk. As for the client side, during the configuration process you’ll be given a script to paste into your web site (which does not have to be portal, in case anyone is wondering) and the chat widget will magically appear.
How can I install this on D365 on premise?
AFAIK, it’s only available for online deployments. The product itself, web chat, can be used without Dynamics 365 but integration is distributed via AppSource and integrates only with online.