Basic syntax is

var obj = Navigator.Find("//input[@id='username']");

In RVL

Here is an example of using it in real-life scenario

https://github.com/Inflectra/rapise-powerpack/blob/master/LoginMicrosoftOnline/LoginMicrosoftOnline.js

https://github.com/Inflectra/rapise-powerpack/tree/master/LoginMicrosoftOnline

To learn how to work with dynamically created objects in RVL please refer to this KB:

Accessing DOM elements in RVL