I strongly encourage the use of the Helper Libraries. They are documented at Helper Libraries for openHAB Scripted Automation — openHAB Helper Libraries documentation. In particular pay attention to the “But how do I…” page. Also look at the library source code which is informative.
I’ve posted some further examples at openHAB 3.0 my getting started notes: Rules (I return and update this periodically so check back often) and much older examples at Experimental Next-Gen Rules Engine Documentation 1 of : Introduction which have a lot of stuff that is no longer correct but is still largely useful. Full docs for ECMA Script have not been built yet.
I’ve not tried to send a notification yet so I cannot say for sure. But org.openhab.io.openhabCloud.NotificationAction is the correct path according to the code on github.